There are any number of tools available online for group collaboration. Here are some links to a few:
Google Sites is a free service that allows you to build very customizable collaborative sites. You can make Sites work almost any way you want: as a blog, as a wiki, as a wireframing system.
Wet Paint. A nice, free, easy-to-use Wiki system.
Google Calendar. You can share a calendar among groups–great for scheduling meetings or making sure that everyone is on the same page as far as deadlines go.