Grouploop
One of the best things about the web in general is the availability of free services.
GroupLoop.com offers once such service. For businesses, groups, or even families that have a difficult time keeping in touch, an account with grouploop can really help.
Imagine that you want to collaborate with five other people but you don't own your own web space and everyone has different agendas. The director of your group can set up an account with grouploop (for free) and post messages, files, calendar events, and such for everyone to see. Want only a few people in an expanded group to see certain events? That's possible too.
I remember several years ago using something similar with yahoo groups, but grouploop's design, interface, and feel are cleaner and easier to navigate. I suggest at least a preliminary try.
