Cosette Paneque points out the lurking has many benefits for all members of the group, and if you wanted to, offers some suggestions on getting them to participate.
Most users – about 90% – don’t participate very much. About 9% of users contribute from time to time. And only about 1% of users participate heavily and account for most contributions. This is known as the 90-9-1 rule and the phenomenon is known as participation inequality.