Invite users to group with minimal access

Proposal

On GitLab.com, it would be beneficial for organization to be able to invite users to their groups with the default role of minimal access. Right now, the lowest permission level is Guest.

Some organizations might want to default to giving users/external collaborators minimal access at the top-level group to avoid exposing company information that Guest users have access to, like non-confidential Epics created at the top-level group.

It currently isn't clear in the documentation that users must first be invited using a different role, and then their role switched to minimal access.