See it as a problem. Help the group see that passivity doesn’t solve problems; it causes them. For example, say, “Mark, when you don’t open up with us, it cuts us off from you and people wonder who you really are.”
Help them with their language. Passive people often use passive vocabulary, such as, “He made me … she wouldn’t let me … they stopped me from….” This language re-creates the problem in their heads. Bring this up, and show them how to use active (and more reality-based) language: “I allowed him to … I let her … I caved in when they….” In fact, this is a helpful exercise for most group members, as it is easy to slip into passive vocabulary.
Remind and internalize. Have the group help the person by reminders, and then see if they can internalize the process. You might say, “Claudia, why don’t we call on you when you are withdrawing? We’ll check in with you so you can safely tell us how you are doing. And after a while, we’ll see if you can volunteer on your own without our nudging.” If these are safe and good experiences and the person understands the issue, this approach can work well.
Passive people are less trouble in a group than noncompliant people, but don’t let them slip away. Many have terrible life problems because of their passivity, so make sure you are engaging that as an issue.
Cloud, H., & Townsend, J. (2010). Making small groups work: what every small group leader needs to know. Grand Rapids, MI: Zondervan.