Not sure about this case, but iirc you can have contributors to a project without them owing "part of the copyright" for the stuff they wrote. You just have to make them assign the copyright to you if they want to include their code in the project (they can always fork if they don't like that, but the original authors still get the copyright to the core project).

Also, and orthogonal, a lot of time there are 1-2-5-10 core devs, and the rest 100s are just some small changes here and there, fixes to the documentation, some plugin contribution, etc. In other words, easily writen out, if it comes to that.