How do I use Google Play Game to login into the Multiplayer?

I created a pong like game and I was thinking about using the Pong Multiplayer Example to implement this feature in my game.

But I want to login in the multiplayer using Google Play Game Services, and I'm stuck at this point.

Anyone know how to solve this problem?

Thanks in advance

I don't see what the problem is. First log in to Google Play Game Services, then join a room in the multiplayer object.