For The Last Days mod, the way we acquire new developers is by seeing which ones actually get stuff done. We get offers of help all the time, but until they turn up with the goods we are pretty non-committal as far dev-status is concerned.
I had contributed for over a year before AncientWanker invited me to the dev board and inner team.
That's pretty much my attitude as far as these things go. Now, I won't lie; personally, I work alone 90% of the time, and TLD is the only project I've really collaborated on. The Hundred Years War did have contributors, but it was the same thing: several people offered to help, so I gave them the necessary direction and waited until they turned up with the promised content, or didn't. In neither case was I in danger of loosing massive parts of the mod, simply because I gave them only the files they needed to work with.
Miserly? Perhaps, but it seriously cut down on the confusion. When a writer volunteered to write some characters, I gave him the few files he needed, he edited them and sent them back. Integrating them back in was a cinch.
For mods with lots of heavy scripting, the answer isn't much different. Depending on the number of collaborators, after you have found a trusted scripter, you set up an SVN version control system or some such that house the full game code/module system, and set up a few rules that must be followed. Such as no committing of work with bugs in it, no committing code that causes the game to crash, etc. Those rules are simply to prevent one person from having to fix another person's code and/or bugs, which is a *****.
If the people won't agree to these rules, you don't want them on your team in the first place. But presumably, if they do have access to your files, it's because they've already shown initiative and helped substantially.
Alternatively, if you have a dedicated scripter, he can do all the scripting and send you the mod creator/coordinator periodic updates for you to work with.