Overall: My vote goes to using pre-existing tools like Google Groups, HN Userbase to get started

If a component of the existing tools requires a paying element, perhaps a very low joining fee could be in order. Taking money always changes how people react of course.

e.g. USD$1 to join --> $400 kitty to spend on a paid-hosted plan

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If you need to build something out, consider:

http://pinaxproject.com/ A python and django OOtB framework

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or Tiki... which has an awful lot of features that don't require writing code:

http://info.tiki.org/Tiki+Wiki+CMS+Groupware

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They say:

    * Wikis (like Wikipedia)
    * Forums (like phpBB)
    * Blogs (like WordPress)
    * Articles (like Yahoo News)
    * Image Gallery (like Flickr)
    * Map Server (like Google Maps)
    * Link Directory (like DMOZ)
    * Multilingual (like Babel Fish)
    * Bug Tracker (like Bugzilla)
    * RSS Feeds (like Digg)
    * Free Open Source software (LGPL) 

And much more...

    * all integrated (like no one else)