We have 40 students and use the subteam method as well.
This year we broke into:
-turret, gun, and hood
-Chassis and internal collector
-Bridge handler and external collector
-bridge balancer "stinger"
-Website
-Chairman's Award
-janitorial services
Our system for putting people in groups is to write the groups up on the whiteboard and allow the students to put their name under the group that they most want to work in. Then the mentors will later switch the names around to spread around the more experienced students so there is not one "awesome" group with all the seniors and one lowly group of all freshmen. These groups are then headed by either one or two mentors who control their "minions." Every though we all work in the same shop, in order to maintain continuity between groups at the end of every meeting all the groups meet back together in the confrence room to discuss/ show what they did today and they're goals for the near and distant future. For us this is key to the success our subgroup system.
Regards, Bryan