Add InvitePlayer method to multiplayer server

osu.Game/Online/Multiplayer/IMultiplayerRoomServer.cs

@@ -99,5 +99,12 @@ namespace osu.Game.Online.Multiplayer
         /// </summary>
         /// <param name="playlistItemId">The item to remove.</param>
         Task RemovePlaylistItem(long playlistItemId);
+
+        /// <summary>
+        /// Invites a player to the current room.
+        /// </summary>
+        /// <param name="userId"></param>
+        /// <returns></returns>
+        Task InvitePlayer(int userId);
     }
 }

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -260,6 +260,8 @@ namespace osu.Game.Online.Multiplayer
 
         protected abstract Task LeaveRoomInternal();
 
+        public abstract Task InvitePlayer(int userId);
+
         /// <summary>
         /// Change the current <see cref="MultiplayerRoom"/> settings.
         /// </summary>

osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs

@@ -106,6 +106,16 @@ namespace osu.Game.Online.Multiplayer
             return connection.InvokeAsync(nameof(IMultiplayerServer.LeaveRoom));
         }
 
+        public override Task InvitePlayer(int userId)
+        {
+            if (!IsConnected.Value)
+                return Task.CompletedTask;
+
+            Debug.Assert(connection != null);
+
+            return connection.InvokeAsync(nameof(IMultiplayerServer.InvitePlayer), userId);
+        }
+
         public override Task TransferHost(int userId)
         {
             if (!IsConnected.Value)