[normalmutant]

Sorry none of us answered this sooner.

Our coach tries to include as many new members as possible. What you have to ensure is that the student is dedicated to attending meetings and has a definite role or multiple roles in the team. You don't want him/her sitting around doing nothing during meetings, or not even attending.
Over the summer, we invite those who express interest to attend the workshops that our state runs and training meetings that we run. This will weed out those who are not genuinely interested, and train the rest in their different roles. If a student misses for no good reason, our coach will keep this in mind. This is a good forecast of how diligently they will attend or participate during the school year. We also hold an orientation session so that the parents know how much work it will be and how often they will have to drive their children.

Other than that, we typically try to be inclusive, not exclusive. This year we are expanding to 3 teams.