lots of ways to support the team, such as building up and repairing defenses at your base, defending it when attacked, etc. In fact, I've had a few intense matches that ended in a draw, where we were on our heels nearly the whole time just fighting off the attackers. Sounds sucky but actually it was pretty fun.
I've generally found that there are 2 main roles to play as, but I think personally there is a 3rd. One that I've found I play as, and I always have. Attackers and Defenders are the obvious ones, but the one I found is a Mid, or rather Support. These guys (I'm not the only one that does this) usually go out and build an outpost with a supply base then defend it. Then, wherever they need help, they help. It may sound like your just switching from Defender to Attacker, but actually it's a whole new role on its own because you're supporting everyone, like a wing in soccer. The outpost lets your team spawn wherever you place, then with a supply base put onto it, they can get a full loadout before they leave. Mids are your support class if there WERE any classes.