Try to engage your whole team in a non-FIRST related activity. When I was in HS the build team would quite often go to late movies, eat food at The Fort (or your local cheap-24hr-diner) after a build session. We also had numerous off-season get-togethers like LAN parties, so-called "man-parties" (though girls went too

) where we'd paint-ball, shoot potato cannons, putter around on one guy's dirt bike, drive RC cars, etc. The team recently got together for a BBQ to watch one of the US World Cup games.
FIRSTers quite frequently have similar interests, and getting together outside of the pressures and confines of an actual FRC meeting can build friendships and cohesiveness.
As you are in a position of leadership on your team you should also try to get everyone together and write a mission statement for your team. This will help unify and direct your team because everyone will be clear about what is expected from the team as a whole.