Many will join a community of practice just for what they can get out of it. There's nothing wrong with that; presumably it creates value or disseminates new insights or learning for the organisation. Those insights have to come from somewhere though.
As Michael puts it "You want to make contributing to your group irresistible. Don’t expect to write the acceptance message and leave it at that". But that's often what happens. Michael has some practical help and some communication tips and templates that can help make more joiners active rather than passive. You can read his blog on Membership Acceptance here.