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Collaborators can have "Read", "Read/Write", or "Admin" privileges. You can choose after you add them.

I don't have that option. I can see that Teams have it, but as an individual with a private repo, I only have the input for entering the username.

Guess I have to set up a Team/Org, before I can manage permissions.

Whoops, my mistake. You get the option after you add them as a collaborator. I'll fix my comment above, too.