What's the purpose of "collaborators" with Team license?

I have an organization that is billing on the new Team license plan (using 4 licenses out of 5).

I cannot add collaborators to the repos that we host.

When I try I get:
You have reached the collaborator limit for your current account plan.

It would seem according to the docs that I should use collaborators to add additional people for read/write access.

Maybe I missed something? I’m wondering what the point is for collaborators if I cannot add any – or is that idea invalid for some reason on our Team license tier?

Thanks