How do I Make a Team System on Multiplayer?

Hello, sry my bad english.

I am having a problem when I am trying to create a Team System on a MultiPlayer Game.

The Game: You will put your name, select a room or create a new one and PLAY! (Play=Go to "Game" Layout). In the start of Layout you sign to signillying server, etc. And then you recieve a team ("Blue" or "Red" or "Green" etc.)...

But I don't know how to protect the same players that is the same team, someone can help me PLEASE???