Some services are charged on an actual per-user-per month basis, called an Actually Active user.
There is no practical limit to the number of users you can add to the system.
User
A user is anyone that you’ve added to the system
Active User
An active user is someone that shows up on rosters, drop-down lists, etc. These people can log in to the system.
Inactive User
Inactive users can not log into the system and no longer show up in dropdown lists. They are kept in the database for archive purposes, or in case they re-join your organization. This person might be an employee or volunteer that used to work with you but is no longer actively involved. They are generally hidden from view.
Actually Active User
An actually active user logged in at some point during the calendar month. They may have logged in once, or 200 times.
Passively Active User
A passively active user was allowed to log in, but didn’t actually do so. They are not currently using the system, for whatever reason.