Most players, especially the beta players who are already pretty good at the game, aren't playing yet because the game was supposed to be released this Tuesday.
No teams will likely be forming for a while. Once a bit of time has passed and everyone gets into groove with their favorite demigods and gets to know the levels I'm sure clans and teams will start popping up.
That's your best bet for joining one. I hear Demigod is supposed to have clan support, but I'm simply don't know. I plan on joining one myself once I jump through the ropes depending on the kind of clans that crop up as it's simpler to just join a good one.