Is there a way to add an external team as a repo collaborator?
Hi there! Welcome to the Community!
Unfortunately this is not possible - you’d need to invite each individual as an outside collaborator:
Organization permissions and team admin is something we’re working on a lot at the moment, though, so perhaps you’d like to submit a feature request through our official product feedback form so that our product team can see exactly how you’d like cross-organization invites in GitHub to work?