Create mechanisms to manage/control guest users from unknowingly occupy a license seat

Problem to solve

Currently the question whether a user is a guest user is determined by membership to a group or project (see https://gitlab.com/gitlab-org/gitlab-ee/blob/a31d3aac3dc0e3aacda5592679a74857d5c0dea0/ee/app/helpers/license_helper.rb#L67). This has the implication that a maintainer might grant a role to a guest user, thereby upgrading them into a regular user and occupying a license seat. Customers might desire to manage, control and/or prevent that instance-wide.

Further details

(Include use cases, benefits, and/or goals)

Proposal

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Links / references