How do I join a game?
When you enter a game, you'll usually see two teams, yellow and blue. White is not actually a team. "Not On A Team", or NOAT, is just that, sitting on the sidelines until you join a team.
You'll sometimes find yourself looking at the screen like this:
Now, that game hasn't actually started, but if you click on "Join Team", or double-click the color team name bar at the left, you can join it.
Please be patient however - joining a team is not the same as starting a game. Many players like to sit and discuss how the last game went, or they're looking for new commanders to take over the teams. It may even take a few minutes to sort out and balance teams themselves. It's not good sportsmanship to demand the game to start on your behalf.
However, if you find a screen that has a timer above the "Join Team" button, like this:
The game's already started, and you can feel free and join a team to instantly join the action.
Sometimes a commander will not be able to accept a player immediately. They may have an inferior team and are waiting for a veteran player before they accept a newbie, or the other way round and waiting for a newbie before they accept a vet! Or they may be busy in the game defending a base or leading an attack and simply didn't notice your request.
I suggest requesting to join a team and then waiting 30 seconds. If the commander still hasn't accepted you, quit the team, and attempt to join again. This will send a fresh "Player wishes to join your team" message to them. Rinse and Repeat.

