Online reading groups often grow out of chat room groups, bulletin board posters, or members of an e-mail mailing list. Anywhere that people meet and talk is a place to consider looking for reading group members. Another idea is to check with your local library or bookstores to see if they sponsor their own reading groups. These groups often have a book list already made out and they often provide a meeting space. You can choose to attend only for those books you'd like to discuss.
Attendance can be an issue with some reading groups.
You'll want to choose members who are able to commit to reading the books and coming to the meetings. Make sure your members are dedicated to the idea of a reading group, and aren't just looking at it as a social opportunity. The ideal group size is between 8 and 12 members. While commitment is important, not everyone will be able to make it to every meeting. Fewer than 8, and some days you might not have enough for a good discussion.