Groups can be complicated. Groups sizes and formats, how long they are and how often they meet, whether involvement in them is voluntary or not, how groups progress, and members' roles all contribute to group dynamics.
The American Therapy Association and American Specialists in Group Work both recommend interviewing each prospective group participant to see if he/or she will fit into, and benefit from, the group. This interview does three things:
1. Screens out people who are hostile, egocentric or have inadequate ego strength, psychopathic or sociopathic, domineering, or just plain suspicious.
2. Lets potential members get to know the group leader (though a potential member may already be a client in individual therapy).
and 3. Lets the group leader explain the group's goals and format as well as group members' rights.
The group's goal should help determine who is chosen. A diverse group can encourage interaction and new perspectives, whereas more alike members can more easily focus on a specific problem.