Our group is using version control:
http://www.creationkit.com/Version_control
It takes a bit to set up, but saves a lot of time vs other methods (like round robin and manually merging). There's a couple problems we've run into with it though.
1. You can't seem to merge terrain into a master (.esm) file.
2. When you merge quests into the .esm, the sound file names required change from what they were (for the npc dialogue) so we have to go back and rename a bunch of audio files.
Other than that it's saved us a lot of time as far as merging goes.
For me to get it to work, I had to make sure my steam games folder was NOT in my program files, due to permission issues. So I reinstalled steam and had the path be C:/Games/ instead of C:/Program Files/