As you've built a contracting platform targeting the best contributors to open source projects, you're collecting a group of people who are looking for work and are most fit to judge the abilities of others in the software they're expert at. Why not dogfood?

I would say to maybe allow verified contributors/experts to invite others in under their account, and let them work from that context until they have enough feedback that they should be invited to join independently of their previous sponsor. My only real doubt is that even being a known contributor is going to be gamed unless there are clear processes for vetting and distinguishing between even the people who are verified contributors to a project, because some will be experts and some will not be.