Use IN relation when getting members of authentication group to select

When logging in using the mentor group code, I got an empty set of mentors to choose from, despite the fact that there were plenty of mentors 'IN' the 'mentor' group. It turns out that the logic for selecting them found all the participants with other relations than 'IN', which works for students because 'IN' and 'classmates' are the same set, but not for mentors where there are only mentors 'IN' 'mentor' and no other relationships with that group.

This changes the logic to use only the 'IN' relationship, rather than everything else, so that both students and mentors can select themself.