Members join a group because they have similar interests and they want information. That's what you are thinking, right? You offer occasional information, and as a group owner you don't really try to offer your members a sense of belonging. If a member really feels like they belong in a group, the loyalty, commitment and willingness to help increases dramatically.
For a member to feel like they belong requires high-involvement on both parts. More than you are most probably doing at the present.
Every member needs to be treated as an individual. He needs to be welcomed by others, invited to get involved, given responsibilities, have a buddy to help them and seek them out if they are absent for a period of time. To do all of this you need every member to help run the group. You want each member to feel you care about him.