suggestions for web spaces for cross-institution project

I am working with 4 others to plan a symposium – early days yet – but watch this space. Anyway, so far we are using just emails, but I think we will need a shared space for the ongoing communication and the documents. We are all at different institutions and in 2 different countries. I’ve used SharePoint (Unite) for projects here at La Trobe, but I understand that we can’t use that across institutions (plus the Discussion component doesn’t look that friendly for communication)

Google Groups is OK for the communication function, but the space for files and pages doesn’t seem to have the capacity for creating folders and keeping track of versions of files etc.
Any suggestions would be appreciated
Thanks, Meg