Outside of the CK, there's also some very important tools modders for big projects can use to keep things on track.
The first is, look into an SNV/version control system, especially if your mod involves loose files outside of the normal ESP (such as texture, audio, models, etc.). This is less nessicary to do, and probably detrimental if you are just working on an ESP.
For http://www.mechlivinglegends.net, we use TourtoiseSVN ( http://tortoisesvn.net/ ), which is open source and free to use. Of course, someone would need to rent out or pay for a server to put the SVN on, but other than that it's free. Basically how it works is this: you have the SVN server, which holds the main mod files. Every person who has access to this main server has a repository on their own computer that mirrors the server's contents. Whenever someone makes a change or adds something to the server, mod team members can do an "SVN Update" right in windows, where they download the "update" to their computer. Or they can do an "SVN Commit", where they basically upload changes to the master server that other people will download when they update. This might be tricky when working on one ESP, which is where it would be useful to split your ESP up into each "area" that gets worked on by different people (i.e. an ESP for interiors, for code/scripts, etc). Then at the end of the week, you could have a team lead merge all changes to these seperate ESP files, and upload the master ESP.
SVN's are VERY nice because they directly track every change made, which is very useful for debugging or reverting any accidently screw ups. Most SVN's (including Tortoise) have extensive backup abilities that last for a long time. If I wanted to, and I was a mod lead, I could revert the whole mod (or just a specific part of it) to changes that were done for a year ago, and then back again. This makes it really easy to fix broken things - just revert. Or if you accidently delete your file, you'll always have a backup on the main server.
We also use Trac, which is a free project management system: http://trac.edgewall.org/
Trac basically lets you keep track of your projects by clearly laying out bug reports, feature lists, timelines for the project, and being able to create "tickets" (tasks, either for a bug or a feature) and assign them to people with varying priority levels. It's very useful for keeping everyone on the same.. "trac"

I'm pretty sure both of these things require a server to run them on, so you'll either need to rent out a small one or hook up your own. I don't think they need to be particularly powerful, though the SVN can put a big strain on a server if it's a rather low end one and you've got all the fancy features unlocked and it's used heavily.