Well, I am using Construct 2, which doesn't handle multiple persons working on the same project at the same time really well, in fact, not at all lol. It would require merging files generated by Construct itself, which aren't supposed to be edited by hand.
The other problem is that I develop the game on my free time, which I have a lot because I don't have a job at the moment, but this might change in a more or less near future. Also, my work schedule is a complete mess, hell, in fact I don't even have any, one day I work on integrating animations, then the day after I'll be working on a level; and that's the same for the time I spend working on the game everyday, 8 hours on Monday, and nothing on Tuesday because I don't feel like working. So yeah, a complete mess, that would make working efficiently with someone else really hard, both for me, and that person.
BUT, what I was thinking about doing, is to upload demos regularly on the itch.io page, and give away a few download keys here and there to let people download the demo. That way, I hope I'll get some feedback, and people will be able to participate in shaping some aspects of the game if they want to. Although I'll still have the final decision of course, when creating a game, you gotta take user feedbacks into account only up to a certain point, or else you'll end up making another bland, boring, uninteresting game because you're trying to please too many different kind of players :)