1). Ensure there is a termination experience; it is not uncommon for group members to leave with little, or no notice. A specific group agreement around termination is necessary (one that is established at the outset of group). This agreement can be somewhat flexible depending on circumstances when a particular member is in the process of leaving. A reminder of the termination policy can help provide motivation to say goodbye, even if the departing member doesn't stay for the original agreed upon number of sessions. Encourage all group members to talk about their relationship to goodbyes and the experiences they have had saying goodbye; to this point expect some resistance to goodbye in general. If you have a guideline such as four weeks notice to terminate, expect additional resistance and sometimes eye rolling directed at the group leader.
2). As appropriate, explore the reason for leaving and don't hesitate to challenge the group member to stay. Group leavings can often be reactions to the difficulties of group interactions, but often are stated as a practical matter, such as finances and scheduling. Suggest: "If this practical matter weren't the case; what would make it hard for you to stay in group?" This "what if" exploration can highlight hidden material about how the member really feels about the group.
3). If the group member still wants to leave (resisting apparently your wonderful persuasions!), make sure the member has an effective, reflective, and positive goodbye.
Even for members who have been challenging, it is important that they hear what they have accomplished and what was positive about their experience. Both in their eyes, and from the perspective of the rest of the group.
4). If the group member "bolts", that is, leaves the group with no notice, lead a discussion about this members' departure including the ability to have initial (and perhaps over time, reflective) closure. This may include anger about the member leaving, disappointment, relief, and loss of connection, as well as reflection on the Leader's role in not being able to keep the said member in group.
5.) Towards the end of a regular goodbye, ask group members what they would say about "Sally" after she's left, and ask them to consider saying it now while she is still here. This exploration emphasizes intimacy and challenges the normal habit of talking more candidly about people when they are out of sight.
6). Always ask the group the week after a member has left, how they are feeling about the member not being in group and what, if anything, is unfinished. I usually wait midway through this particular session before mentioning this, in case a member brings up the topic before I do.