I think the point about a greater focus bringing people in is true. The more specialist you make it the more likely to hit a sweet spot where people think -'ah that's just what I need right now'.
On the other hand, I think that this also tends to work well as things then are targeted at something that could benefit the individual in terms of their professional development. I think that kind of professional networking and specialist knowledge is what a space like MadLab is good at.
But I think that then comes at a cost in terms of broader outreach, to a non-technical public which is more what I was thinking of in the original aims of this post.
Not saying software user groups are a bad idea for the space at all, just not my focus.
Also, I think another way of pulling people in is doing something with their kids or grand-kids. Serves 2 purposes, a bonding activity and increasing general tech skills of their offspring (future earning power to pay for their retirement homes).
I realise I'm in danger of just moaning rather than getting on with it, ie. doing something and just seeing what happens! I will do 'something' but would much rather do it as a broader coalition of people than just me. As @9600 says doing it for longer regularly does have positive effects.
Also like @treb0r I'm a bit time poor but one evening most month will be doable for sure.