I'm not involved in the organising side of things, but the most successful sessions I've been to are the ones which are very clear in all their publicity about what their aim is, and have organisers who are happy to explain loudly to anyone who asks (eg people who wander into the pub and ask what's going on) so that everyone is reminded of the 'ethos' on a regular basis. It also makes them more inclusive, to my mind, in that newcomers know more or less what's expected and can feel fairly confident about whether or not their contribution will fit the bill.

Also, they started off with the organisers inviting a core of like-minded friends to join them. If there's not enough interest to achieve that, it's a fairly clear sign that there's not a sufficient gap in the market for your proposed club.