Collaborating with a cousin was the way I learned most about research & is one of the quickest way to get things done. Google has made this easier for you with Documents, Spreadsheets http://docs.google.com/ & new to Google Labs “Note Book” http://www.google.com/notebook/. Everyone must have a Google account, but signing up is free.
Documents: I don’t see a lot of good unless several people are writing a family history together. But it does have nice features, export as HTML, RTF, Word, OpenOffice & PDF. You can Insert Images, Links, Comments, Bookmark, Separator or Special Character. View what revisions have been made. Choose who can views or who can make changes. Publish to the web (a unique URL on Google) or to your Blog. It has some of the same features you will find in a Word Processors, but personally find it a little bit limiting. It will solve the problems of people using different Word Processors.
Spreadsheets: I don’t use spreadsheets very often so I’m no expert. But if you are transcribing something like a cemetery, indexing or anything you might need the use of a spreadsheet, this can be very useful. Each of you can take a portion of what you are doing and work on the same project together.
Note Book: I really like this one, forget about bookmarks you can a select section of a page & come back to look at it later (images are also saved). It requires a little plugin on your browser, but after that is installed all you do is highlight the portion of the page you want, right click and select “Note This” or click the new little notebook icon on the right side of your browser & select “Add Note”. It won’t do PDF files on the web, but it does let you type or copy/paste the text & have a link to that file. I did find a problem with long pages, seams to have some problems with pages that would be much over 2 pages in my Word Processors, and Netscape browser doesn’t seam to be supported.