Share results with the group -- participants gave their time to work with you and give you their opinions, the least you could do is provide them with feedback This can be done through mail, phone, or email. Sometimes it's even possible to bring the group back for a second session, to review results, verify their accuracy, and/or explore other themes.
Perhaps some members have now become more interested in the issue and would like to get more involved. Consider offering them an opportunity. Focus group could be an indirect recruiting tool.