I'm setting up an Ubuntu server that will have 3 or 4 VirtualHosts that I want users to be able to work in (add new files, edit old files, etc.). I currently plan on storing the sites in /var/www but wouldn't be opposed to moving it.

I know how to add new users, I know how to add new groups. I'm unsure of the best way to handle users being only able to edit some sites. I read over the answers here in this question, so I was thinking I could setup a group and add users to that group, but then they'd all have essentially the same permissions. Am I just going to have to assign each user specific permissions? Or is there a better way of handling this?

Added: I should also note, that I'll have each user login in via SSH/sFTP. The users would never need to do anything else on the server.