People don't want to be good at software. They want to be good at fun things like acting, writing, and ultimate frisbee.See also: Social Media for Social Change Behind the Nonprofit Firewall (and the discussion in the comments).
Once you identify the areas where the software can improve the theatre folks life, you’ll have a much easier time convincing them to give it a shot. So in their mind they won’t be using "social network software", they’ll be using a tool to help them be a better theatre group.
This is an unfortunate side-effect of the social networking craze. We have new words that we're using to communicate among those of us who design the software, but for the vast majority of folks who will actually use the software, the terms don't mean very much. So while you may understand what I mean by "niche social network", the people actually in the niche social network think of themselves as performers, actors, or what-have-you.
The issues are a bit different for social networks - if you get it right then your users are your content creators, while you'll probably need others outside of IT to contribute if you want blogs or videos or photos about your organisation.
Finding real world metaphors also seems to help - Andy Powell described the Ning site for the Eduserv Foundation Symposium 2008 as "a virtual delegate list - a place where people could find out who is coming on the day (physically or virtually) and what their interests are". This description has made a lot of sense to people I've discussed it with - everyone knows what a conference delegate list looks like, and everyone has probably also wondered how on earth they'll find the people who sound interesting. A social network meets a need in that context.