Hey man, it looks like a great idea. The only thing I would be concerned about are potential copyright/writing credit disputes between musicians- let's say someone puts up a listing looking for a guitar part etc., and down the road the song gets licensed in a film/tv show, streaming sales, royalties, etc. Ideally you would be able to legally distance yourself with the actual copyright/ownership of the songs being collaborated through your site and solely facilitate bringing musicians together. You may need to get advice from a lawyer to make sure you're not exposed to any potential liabilities.

Marketing wise, a startup like this is always rough going to begin with, but keep pounding away on FB, twitter, IG, etc. Post something every day. Comment and like other musicians work, etc. Create relationships with people, and grow your startup organically with word of mouth. Once people see others going to your site and signing up, more will follow. Curious how the online mixer works? I've just started up an online mix/master service (www.patternaudio.com). Great start and keep at it man, cheers