Ask them to join your group. Choose people with similar educational goals but different back-grounds and methods of learning. You can gain from seeing the material from a new perspective. You don't have to make an immediate commitment. Limit groups to five or six people.
Larger groups are unwieldy.
If that session works, plan another. After several successful sessions, you can schedule regular meetings. Another way to get into a group is to post a note on a bulletin board asking interested students to contact you. Or pass around a sign-up sheet before class. The advantage of these methods is you don't have to face rejection.