Some NEMO stuff on the subject of mentors that might help:
A Mentor Is... and
Recruiting and Retaining Mentors.
If your team organization model allows for it, treat the team like a business. Define your goals; determine what types of "employees" you are looking for to successfully meet those goals; advertise (recruit) people with those skillsets or those looking to learn those skills; interview likely candidates. Have a performance appraisal system in place so that each employee's strengths and weaknesses can be discussed in a nonconfrontational setting. Is there something the team can do to help the person succeed? And have consequences in place for if it's just not working out. Do you keep an employee who is clearly alienating all the other employees and who doesn't want to change? I know this sounds extreme, but most students in FIRST will be in the workplace in 4-8 years. And most companies have some sort of system like this in place, so why not adopt this model?