Based or the Subscription License you purchase you may have a limit on the number of Active Users. You are only permitted to have the number of Active Users that you have purchased. You are permitted to have more Users in the system than you have purchased licenses for but the rest of those Users will automatically be Inactive.
For example, if you purchase a 5 User License you can have 5 Active Users at any one time. You could add a total of 10 Users. Users 1-5 would be Active and Users 6-10 would automatically be Inactive. If you wnated to make User 6 Active, you would need to go in and Deactivate one User and then you will be able to make User 6 Active. See the Managing Users section for how to change the Active setting on a User.