Very lightly edited email to a friend who asked about building momentum in a new academically-based hackerspace, posted here in the hopes it might be useful to others.
First, timing. You wrote your email midsummer -- I'm not sure how much that has to do with it, because summers are usually planned in advance (in a university context) and it's difficult for a substantial chunk of people to add in new commitments mid-cycle. Now it's the start of the school year, where everyone's having their callouts -- probably easier to pick up new people (but also likely to have a high washout rate the first few weeks).
Also make sure that -- if you don't already have this -- your space has a drumbeat that's extremely visible. The spaces we're in have a rhythm; when we build spaces within a space (groups/clubs/labs within a university, in this case) we also need to set a rhythm -- it's calming, reassuring, lets people know things won't disappear if they look away, lets them know they can come back later. (You may not see upsurges in participation until year 2, 3, 4, when people go "ah, yes, they've been around long enough that they're not going to go away; my time invested there won't be lost.")
Transparency. Allow people to eavesdrop on your activities and thinking as much as possible without revealing themselves unless they want to. For instance, blogging (one common way folks do this) or having an in-space computer dedicated to a twitter account anyone can walk out and shout out things on -- new projects and notes and quotes stuck up on bulletin boards around the space keeping the news and "who's working on what" info a fresh stream, making accidental connections more likely. Become a journalist; interview people, post their notes and quotes and work up, connect them with others.
And be visibly messy and incomplete. For instance, if you're blogging, blog about confusion, dead ends, open questions... let folks watch the others in the space struggle, instead of only putting out perfect projects. Put out mistakes -- not just because it helps others help you fix them, but because it also reassures them that they don't have to be perfect to jump in, that y'all ask questions that are as "stupid" as the ones they have.
Bar to entry -- you may have come across the term "legitimate peripheral participation" -- look up the wikipedia article on "Communities of Practice" if you haven't checked out Wenger's work already and you'll likely get a kick out of it. Folks can't necessarily easily add new commitments mid-cycle, so how can you make the commitment barrier lower, how can you get it so they can make a visibly meaningful contribution in 5 minutes with no further obligations? Think of how -- I think it was in my interface design class that we had a discussion about how good unsubscription, warranty, reset, etc. policies up-front encouraged more people to buy, install, etc. More people will make longer-term contributions down the line if they're initially reassured they don't need to make commitments -- instead, the little contributions (with no further obligation) add up until you look up one day and go "wait, I've been there every Thursday for the past 7 months!" So maybe experiment with not allowing people to make commitments, somehow. Design things so that they can't be continued, and see if that subtle psychological effect pays off.
Leaving things undone -- sort of a corollary to the mess. Don't finish things! This will be painful. Publicly, deliberately, abandon something that's 90% complete when the 10% to finish it is so obvious, so easy, and will make such a dramatic difference to the finished product -- and cheer loudly when someone discovers that, finishes it, and give them lots and lots of credit. A few times, do the slog and let somebody else do the glory -- give them that adrenaline rush and get them hooked, then show them how they can do the slog and become more independent and able to get that glory moment without relying on you. Start things and don't finish them, and be a squeaky wheel about the opportunities -- and make sure that those opportunities are really the fun parts, the cool stuff, not "can someone else do the slogwork for me"?
Have lots and lots and lots of individual interactions with people. Private ones, before public meetings. Prime the pump. There's a tactic you might know for bringing up a controversial proposal at a meeting -- you have individual talks with each other representative about that topic, before the meeting -- and then at the meeting, you let them talk about your topic. Oftentimes the stuff you've talked with them about will pop out, and they'll own those thoughts as their ideas. Same thing here -- if you talk separately to a bunch of people about how their interests might hook up with each other, about the sort of atmosphere you're trying to create at the gathering, and then actually bring them all together in the room, that dynamic often does end up actually happening.
Amplify what folks are already doing, already interested in -- instead of "start this new project with us," look for "hey, you're working on foo, would $thing-we're-doing help with that?" Point out how participation would add to their existing work with very little effort, rather than how cool this new thing is (yep, it's cool, but most folks don't have time for another item in their lives no matter how cool it is). Get folks to perceive it as something they already are/do/have. For instance, "You're teaching freshman electronics next semester, right? Well, Tanya is doing an Intro to the Lilypad thing next week and she's got a fun way of explaining what the different components do, that you might enjoy -- want to come?"
Hope that helps.