diff --git a/Shogi.Api/Controllers/SessionsController.cs b/Shogi.Api/Controllers/SessionsController.cs
index 1b545c7..1d167c4 100644
--- a/Shogi.Api/Controllers/SessionsController.cs
+++ b/Shogi.Api/Controllers/SessionsController.cs
@@ -15,134 +15,146 @@ namespace Shogi.Api.Controllers;
 [Authorize]
 public class SessionsController : ControllerBase
 {
-	private readonly ISocketConnectionManager communicationManager;
-	private readonly IModelMapper mapper;
-	private readonly ISessionRepository sessionRepository;
-	private readonly IQueryRespository queryRespository;
-	private readonly ILogger<SessionsController> logger;
+  private readonly ISocketConnectionManager communicationManager;
+  private readonly IModelMapper mapper;
+  private readonly ISessionRepository sessionRepository;
+  private readonly IQueryRespository queryRespository;
+  private readonly ILogger<SessionsController> logger;
 
-	public SessionsController(
-		ISocketConnectionManager communicationManager,
-		IModelMapper mapper,
-		ISessionRepository sessionRepository,
-		IQueryRespository queryRespository,
-		ILogger<SessionsController> logger)
-	{
-		this.communicationManager = communicationManager;
-		this.mapper = mapper;
-		this.sessionRepository = sessionRepository;
-		this.queryRespository = queryRespository;
-		this.logger = logger;
-	}
+  public SessionsController(
+    ISocketConnectionManager communicationManager,
+    IModelMapper mapper,
+    ISessionRepository sessionRepository,
+    IQueryRespository queryRespository,
+    ILogger<SessionsController> logger)
+  {
+    this.communicationManager = communicationManager;
+    this.mapper = mapper;
+    this.sessionRepository = sessionRepository;
+    this.queryRespository = queryRespository;
+    this.logger = logger;
+  }
 
-	[HttpPost]
-	public async Task<IActionResult> CreateSession([FromBody] CreateSessionCommand request)
-	{
-		var userId = User.GetShogiUserId();
-		var session = new Domain.Session(request.Name, userId);
-		try
-		{
-			await sessionRepository.CreateSession(session);
-		}
-		catch (SqlException e)
-		{
-			logger.LogError(exception: e, message: "Uh oh");
-			return this.Conflict();
-		}
+  [HttpPost]
+  public async Task<IActionResult> CreateSession([FromBody] CreateSessionCommand request)
+  {
+    var userId = User.GetShogiUserId();
+    var session = new Domain.Session(request.Name, userId);
+    try
+    {
+      await sessionRepository.CreateSession(session);
+    }
+    catch (SqlException e)
+    {
+      logger.LogError(exception: e, message: "Uh oh");
+      return this.Conflict();
+    }
 
-		await communicationManager.BroadcastToAll(new SessionCreatedSocketMessage());
-		return CreatedAtAction(nameof(CreateSession), new { sessionName = request.Name }, null);
-	}
+    await communicationManager.BroadcastToAll(new SessionCreatedSocketMessage());
+    return CreatedAtAction(nameof(CreateSession), new { sessionName = request.Name }, null);
+  }
 
-	[HttpDelete("{name}")]
-	public async Task<IActionResult> DeleteSession(string name)
-	{
-		var userId = User.GetShogiUserId();
-		var session = await sessionRepository.ReadSession(name);
+  [HttpDelete("{name}")]
+  public async Task<IActionResult> DeleteSession(string name)
+  {
+    var userId = User.GetShogiUserId();
+    var session = await sessionRepository.ReadSession(name);
 
-		if (session == null) return this.NoContent();
+    if (session == null) return this.NoContent();
 
-		if (session.Player1 == userId)
-		{
-			await sessionRepository.DeleteSession(name);
-			return this.NoContent();
-		}
+    if (session.Player1 == userId)
+    {
+      await sessionRepository.DeleteSession(name);
+      return this.NoContent();
+    }
 
-		return this.StatusCode(StatusCodes.Status403Forbidden, "Cannot delete sessions created by others.");
-	}
+    return this.StatusCode(StatusCodes.Status403Forbidden, "Cannot delete sessions created by others.");
+  }
 
-	[HttpGet("PlayerCount")]
-	public async Task<ActionResult<ReadSessionsPlayerCountResponse>> GetSessionsPlayerCount()
-	{
-		var sessions = await this.queryRespository.ReadSessionPlayerCount();
+  [HttpGet("PlayerCount")]
+  public async Task<ActionResult<ReadSessionsPlayerCountResponse>> GetSessionsPlayerCount()
+  {
+    var sessions = await this.queryRespository.ReadSessionPlayerCount();
 
-		return Ok(new ReadSessionsPlayerCountResponse
-		{
-			PlayerHasJoinedSessions = Array.Empty<SessionMetadata>(),
-			AllOtherSessions = sessions.ToList()
-		});
-	}
+    return Ok(new ReadSessionsPlayerCountResponse
+    {
+      PlayerHasJoinedSessions = Array.Empty<SessionMetadata>(),
+      AllOtherSessions = sessions.ToList()
+    });
+  }
 
-	[HttpGet("{name}")]
-	public async Task<ActionResult<ReadSessionResponse>> GetSession(string name)
-	{
-		var session = await sessionRepository.ReadSession(name);
-		if (session == null) return this.NotFound();
+  [HttpGet("{name}")]
+  public async Task<ActionResult<ReadSessionResponse>> GetSession(string name)
+  {
+    var session = await sessionRepository.ReadSession(name);
+    if (session == null) return this.NotFound();
 
-		return new ReadSessionResponse
-		{
-			Session = new Session
-			{
-				BoardState = new BoardState
-				{
-					Board = session.Board.BoardState.State.ToContract(),
-					Player1Hand = session.Board.BoardState.Player1Hand.ToContract(),
-					Player2Hand = session.Board.BoardState.Player2Hand.ToContract(),
-					PlayerInCheck = session.Board.BoardState.InCheck?.ToContract(),
-					WhoseTurn = session.Board.BoardState.WhoseTurn.ToContract()
-				},
-				Player1 = session.Player1,
-				Player2 = session.Player2,
-				SessionName = session.Name
-			}
-		};
-	}
+    return new ReadSessionResponse
+    {
+      Session = new Session
+      {
+        BoardState = new BoardState
+        {
+          Board = session.Board.BoardState.State.ToContract(),
+          Player1Hand = session.Board.BoardState.Player1Hand.ToContract(),
+          Player2Hand = session.Board.BoardState.Player2Hand.ToContract(),
+          PlayerInCheck = session.Board.BoardState.InCheck?.ToContract(),
+          WhoseTurn = session.Board.BoardState.WhoseTurn.ToContract()
+        },
+        Player1 = session.Player1,
+        Player2 = session.Player2,
+        SessionName = session.Name
+      }
+    };
+  }
 
-	[HttpPatch("{sessionName}/Move")]
-	public async Task<IActionResult> Move([FromRoute] string sessionName, [FromBody] MovePieceCommand command)
-	{
-		var userId = User.GetShogiUserId();
-		var session = await sessionRepository.ReadSession(sessionName);
+  [HttpPut("{name}/Join")]
+  public async Task<IActionResult> JoinSession(string name)
+  {
+    var session = await sessionRepository.ReadSession(name);
+    if (session == null) return this.NotFound();
 
-		if (session == null) return this.NotFound("Shogi session does not exist.");
+    if (string.IsNullOrEmpty(session.Player2))
+    {
+      session.AddPlayer2(User.GetShogiUserId());
+    }
+  }
 
-		if (!session.IsSeated(userId)) return this.StatusCode(StatusCodes.Status403Forbidden, "Player is not a member of the Shogi session.");
+  [HttpPatch("{sessionName}/Move")]
+  public async Task<IActionResult> Move([FromRoute] string sessionName, [FromBody] MovePieceCommand command)
+  {
+    var userId = User.GetShogiUserId();
+    var session = await sessionRepository.ReadSession(sessionName);
 
-		try
-		{
-			if (command.PieceFromHand.HasValue)
-			{
-				session.Board.Move(command.PieceFromHand.Value.ToDomain(), command.To);
-			}
-			else
-			{
-				session.Board.Move(command.From!, command.To, command.IsPromotion ?? false);
-			}
-		}
-		catch (InvalidOperationException e)
-		{
-			return this.Conflict(e.Message);
-		}
-		await sessionRepository.CreateMove(sessionName, command);
-		await communicationManager.BroadcastToPlayers(
-			new PlayerHasMovedMessage
-			{
-				PlayerName = userId,
-				SessionName = session.Name,
-			},
-			session.Player1,
-			session.Player2);
+    if (session == null) return this.NotFound("Shogi session does not exist.");
 
-		return this.NoContent();
-	}
+    if (!session.IsSeated(userId)) return this.StatusCode(StatusCodes.Status403Forbidden, "Player is not a member of the Shogi session.");
+
+    try
+    {
+      if (command.PieceFromHand.HasValue)
+      {
+        session.Board.Move(command.PieceFromHand.Value.ToDomain(), command.To);
+      }
+      else
+      {
+        session.Board.Move(command.From!, command.To, command.IsPromotion ?? false);
+      }
+    }
+    catch (InvalidOperationException e)
+    {
+      return this.Conflict(e.Message);
+    }
+    await sessionRepository.CreateMove(sessionName, command);
+    await communicationManager.BroadcastToPlayers(
+      new PlayerHasMovedMessage
+      {
+        PlayerName = userId,
+        SessionName = session.Name,
+      },
+      session.Player1,
+      session.Player2);
+
+    return this.NoContent();
+  }
 }
diff --git a/Shogi.Domain/Aggregates/Session.cs b/Shogi.Domain/Aggregates/Session.cs
index 1c59c90..4ae8e02 100644
--- a/Shogi.Domain/Aggregates/Session.cs
+++ b/Shogi.Domain/Aggregates/Session.cs
@@ -4,28 +4,29 @@ namespace Shogi.Domain;
 
 public class Session
 {
-	public Session(
-		string name,
-		string player1Name)
-	{
-		Name = name;
-		Player1 = player1Name;
-		Board = new(BoardState.StandardStarting);
-	}
+  public Session(
+    string name,
+    string player1Name)
+  {
+    Name = name;
+    Player1 = player1Name;
+    Board = new(BoardState.StandardStarting);
+  }
 
-	public string Name { get; }
-	public ShogiBoard Board { get; }
-	public string Player1 { get; }
-	public string? Player2 { get; private set; }
+  public string Name { get; }
+  public ShogiBoard Board { get; }
+  public string Player1 { get; }
+  public string? Player2 { get; private set; }
 
-	public void AddPlayer2(string player2Name)
-	{
-		if (Player2 != null) throw new InvalidOperationException("Player 2 already exists while trying to add a second player.");
-		Player2 = player2Name;
-	}
+  public void AddPlayer2(string player2Name)
+  {
+    if (Player2 != null) throw new InvalidOperationException("Player 2 already exists while trying to add a second player.");
+    if (Player1.Equals(player2Name, StringComparison.OrdinalIgnoreCase)) throw new InvalidOperationException("Player 2 must be different from Player 1");
+    Player2 = player2Name;
+  }
 
-	public bool IsSeated(string playerName)
-	{
-		return Player1 == playerName || Player2 == playerName;
-	}
+  public bool IsSeated(string playerName)
+  {
+    return Player1 == playerName || Player2 == playerName;
+  }
 }