When I create study groups in my classes I create a sign-up sheet. Students can create their own groups (no more than 4) and they must fill in their information on the sign-up sheet. This helps me keep track of the groups and also determine students who are not in groups.
I also find it important to give the groups some in-class time to talk with each other. It can be difficult for groups of students to meet due to their class schedules, work schedules, and family lives.
Douglas College distributes a informative document on creating effective study groups. The information is geared towards students but as instructors we could promote this information in class.