It’s up to you to decide which partners will be allowed to submit products. You can invite specific partners to your marketplace, or you can allow partners to register on their own---for example, by setting up a partner registration page on your marketplace. In both cases, any product they submit will go through approval workflows you supervise.
If you choose the invitation-only approach, the first step is to create a Company with Developer access for the partner, and invite their team to join that company as users with the Developer role.
Once they have activated their user account, you can point them to our online help about creating products. Our vendor onboarding checklist provides a handy guide of all the steps.