GitHub is perfect for that, look at Unreal Engine 4's recent "Open Source for Customers" approach.
When you give Epic your github username, they automatically add you to their github "Organisation". They have the project there as a private repo, and now I have access to read and clone it.
If I clone it, I have my own private repo copy of it I can work on, and submit pull requests.
I don't believe you need Enterprise to have this feature, if you check their plans you can see:
Collaborators may fork any private repository you’ve added them to without their own paid plan. Their forks do not count against your private repository quota.