Divisions are broken down by the size of the team.
Recon 1-2 Players
Patrol 3-7 Players
Squad 8-12 Players
Horde 13 - unlimited
This is not the size of actual team just members participating in a single event. Example The Psycho Clowns Posse have 22 players on the roster but only have 8 players at event one. They could enter as a Squad. If the Clowns expected more players at future events they could play as a Horde they would just be under strength at the first event.
Players can play on more than one team during a season just not during same event.