yep

Shogi.Api/Controllers/SessionsController.cs

@@ -108,6 +108,18 @@ public class SessionsController : ControllerBase
     };
   }
 
+  [HttpPut("{name}/Join")]
+  public async Task<IActionResult> JoinSession(string name)
+  {
+    var session = await sessionRepository.ReadSession(name);
+    if (session == null) return this.NotFound();
+
+    if (string.IsNullOrEmpty(session.Player2))
+    {
+      session.AddPlayer2(User.GetShogiUserId());
+    }
+  }
+
   [HttpPatch("{sessionName}/Move")]
   public async Task<IActionResult> Move([FromRoute] string sessionName, [FromBody] MovePieceCommand command)
   {

Shogi.Domain/Aggregates/Session.cs

@@ -21,6 +21,7 @@ public class Session
   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;
   }