How do I create or delete a new user?

You have a contractual user quota.

If you have not reached your quota you can create a user by going to ADMIN /users and clicking on "create user".

If you have reached your quota, you can free up a space by deactivating one of your user. To do thia, go to the ADMIN /users to modify the user concerned and leave only the "Deactivated" right ticked. The history of his actions will be kept.