Is there software to install on an ubuntu server, and then use to collaboratively edit text files, sometimes adding some attachments? it's important that no data is lost if 2 people save the same doc with different contents at the same time. I've found seafile, but it seems too complicated considering it need 3 ports to work, and requires manual installation.
Try Etherpad, a nice alternative to Google Docs based on Node.js. We used it in some projects with relative success.
ownCloud is an open source, private cloud server, like dropbox or google drive, but you can install it locally. They recently added a collaborative document editing feature that you can see in action at the top of this page: http://owncloud.org/six/
I have only briefly tested the document editing aspect, but the file archival aspect has worked great across multiple machines and platforms. Just like with dropbox, you always retain a local copy of your files.
You could try http://www.collatebox.com/ , or simply write to them to kow if they have a solution for that.