Ed Stetzer writes a post about encouraging involvement in small group ministries:
Through teaching biblically, promoting incessantly, and leading organizationally, we can encourage our brothers and sisters to get involved with small group ministries.
Promote it incessantly
People need to hear about small groups all the time. The more you can promote it, the more likely they are to understand the importance of small groups. I would try to mention small groups in some form or fashion in every service. An ongoing, incessant discussion of small groups is key.
Read the post here.