In general you have to push L3 (The left thumbstick) to talk to everyone in the team.
In addition...
When you get into a game, you can also pick a channel/squad say, Tango, X-ray, etc... (There are 6 to 8 of them, I think) for your own little group in the same team. Once you do that everyone in the same channel will hear you right away.
EDIT: beaten by liverkick. ... and yes, you also spawn near your channel/squad mate.
And then I started to find my rhythm and now I rarely die. Instead, I am just downright amazed at the different environments, and the depth of the physics. It's not just bits of little Chimera that fall and keep rolling around a little afterwards, or that the Chimera die in all sorts of positions, but should they happen to die and get stuck hanging down somewhere, you can shoot or hit them and they will start slipping down and fall convincingly too. And Chimeran architecture becomes a factor a lot sooner in the game than I thought (as did the tank bit). As far as the aiming and shooting is concerned, I suddenly just 'got it' and now my aiming is better than in any other game I've played previously. 