Give every new member a small job to tackle when they join.
This gives all members ownership over a tiny piece of your community. It gives new members the chance to feel involved and build early relationships. It gives new members a taste of early success.
You might task a new member with rounding up the latest industry news. Or you might ask one to create an OPML file featuring you all the blogs of your members.
You might ask them to moderate the newcomers forum or translate your content to an overseas audience.
Perhaps you can give them the arduous role of talking to a group, spotting a problem, and giving the job of solving it to the next new member?