ISO: collaboration software

I'm working with a small group of volunteers to re-engineer business process for a local charity. We primarily need the ability to centrally store documents (meeting minutes, SOPs, process diagrams, etc.). Major beneficial options would include discussion threads, calendar, task tracking.

Would be great if I could install it on one of my Mac Minis, but I guess a low cost online hosted option would work, also (remember...we're volunteers working for a charity, lol).

thanks for any suggestions you have -

carl