If a user is not a guest, they should automatically take up a paid seat
Problem to solve
GitLab currently allows users to be added to an instance and not take up a paid seat. Only when a user is added to a project or a group do they take up a paid seat. This causes confusion on the user management page as customers try and decipher who is taking up a paid seat and who isn't and makes it harder when a customer is trying to handle their renewal and process their true-ups with GitLab.
Proposal
Users that are not guest users should automatically take up a paid seat as soon as they are added to the instance, even if they have no logged in yet. The exceptions to this rule should be root admin users, service users, ghost users or support bot users.
If a user is blocked or deleted, they will still take up an active seat so long as they were added after the purchases seats had been reached or exceeded. If they are blocked and deleted before this happens, a seat is freed up for another user to use.
If a user is deactivated, then we should follow the proposal here: #35685
We should do this for self-managed and gitlab.com.