Our team's organization is outlined in our team handbook which you could find here:
http://www.usfirst.org/community/frc...t.aspx?id=7022. This handbook is from 2007 where we had a pretty large team, and some students covered more than one role depending on what they were doing and how much time they wanted to put in. For example, a communications leader could also be on the one of the build subteams. Check out other teams' handbooks for more ideas on organization. Our handbook gets updated every year.
We tried motivating kids by keeping track of how many hours they put in and then judge if they should letter in robotics. Parents then spoke up and the requirements for lettering changed so it's not based on hours but on other requirements. We have not kept track of hours since because some team members weren't completely honest on the sign in sheet and not everyone who was there logging hours was productive. I would not recommend logging hours or offering letters to students in robotics. Very few teams offer letters and being on a FIRST team should pay off enough. Kids should already be motivated to build the best robot they can, and learn as much as they can. They shouldn't need extra incentives.