Or it can involve creatively soliciting opinions and ideas from participants. Seek out important new participants. For instance, if your group focuses on some aspect software engineering, personally invite important or otherwise interesting programmers to join the discussion. You may be surprised how many will take you up on it. Praise people for making particularly valuable contributions.
Participants love to hear from the discussion group manager that their contributions are valued.
Usually it's best to deliver these compliments via private e-mail. Make it clear what should be handled privately. Every once in a while, someone will post a message to a public discussion forum that really should have been handled by private e-mail. This kind of stuff has the effect of cluttering up the discussion group. If it happens rarely, address it through private e-mail.