I know what it's like to not want others modifying your code. What about setting up a repository and giving some trustworthy people read access. Then, they can fork off of it and make their own commits, and send a pull request when they're ready to submit something back to you. That way, you can get the advantages of other developers, while still keeping control over your code.
I'd love that, honestly, but it'd be hard to keep formatting and logic consistent, keeping track of who's doing what. I'd need to clean up some parts, too.
I might, though. After things get stable I'll do some housekeeping and break script objects into class files, refurbish the saving system, etc.
For work we use a repository system and something called TortiseSVN to keep the hivemind collective despite thousands of miles of distance between the 3 of us.
It'd also be hard to decide who to let in. I'd have to be certain of their skills and ability, not just trust. Object-Oriented Programming is an absolute mega-must. Ephialtes and SpaceGuy jump to mind, but neither of them would want to work on CityRPG, lol. Not even sure if SpaceGuy still plays. I'm not familiar with any other developers because of 2 years of being out of the development circle.