It is important to set expectations ahead of time. Before guys even apply, they need to know what is to be expected if they join the group. Once the group begins and they’ve signed the covenant, they should be held to the covenant. Holding guys to the covenant early on, coupled with the positive peer pressure from the other mentees typically solves most issues you may encounter in the group. 

If an issue does arise, it’s best to pull the mentee aside one-on-one and attempt to address it. The key is to be a velvet hammer, firm but fair. In the rare case, where the issue persists and the mentee is refusing to engage with the group or to do the work or is constantly distracting the group, or taking the conversation in the wrong direction, you might have to dismiss him from the group.