Actually listen to the joinable? function

apps/amethyst/lib/apps/game_coordinator.ex

@@ -96,7 +96,7 @@ defmodule Amethyst.GameCoordinator do
   end
 
   defp _find(type, state) do
-    case state.games |> Enum.find(fn {_, game} -> game.mod == type end) do
+    case state.games |> Enum.find(fn {_, game} -> game.mod == type && game[:mod].joinable?(game[:refs]) end) do
       nil -> {nil, state}
       {_, game} -> {game, state}
     end

apps/example_game/lib/example/game.ex

@@ -20,4 +20,9 @@ defmodule Example.Game do
     Logger.info("Player at #{inspect(from)} moved to #{x}, #{y}, #{z}")
     :ok
   end
+
+  @impl true
+  def joinable?(refs) do
+    true
+  end
 end