If you create a user, they get assigned to the jira-users group which apparently they have to be part of to log in to the system.
Being part of the Jira users group means the user is then part of the 'users' group across ALL projects. What if you then want to stop the user accessing certain projects? It seems logical to remove them from the users group for the project but you can't - it then removes them from the global Jira-users group meaning the user can no longer log in.
This doesn't seem logical to me?