[Karibou]

First off: are your meetings consistent every week? I would assume so, but you never know. If they're just kind of sporadic, there's your problem.

Second, I'd suggest setting a requirement for the number of hours spent working. Say that starting next week, you must put in X hours at robotics in order to stay on the team. Call a mandatory meeting and give them a reality check, too.

Make sure that if they show up, you have things for them to do. Explain to them why what they're doing is important. People seem to be a lot more motivated to work when they know what they're making or doing.

Finally...rely on those four people and your mentors to do the main work, and assume that's all your team is. If you set your realistic expectations low, you won't be disappointed if more than those few people show up.