Confluence instances are licensed for a set number of users. Administrators frequently ask how to remove one or more users so they can add new users without having to increase the number allowed by their licence. Currently we tell them to effectively deactivate those users by removing the "Use Confluence" permission from them, both directly and via their inclusion in groups. http://confluence.atlassian.com/display/DOC/Removing+a+User

The problem that many many administrators then face is that the number of users actively using the system is somehow cached. (I believe it's their membership of groups that is the problem but I haven't checked).

This means that in order to add new users, they must:

1) remove permissions from existing users
2) wait a while (or clear the caches from the cache statistics page).
3) add new users.