When subscribing to smrtDialer you chose how many user seats (licenses) you want to have available. That practically translates to how many users can use smrtDialer at a time.
You can always assign any user from your smrtPhone account to be able to make calls using the power dialer.
Just go to Menu -> smrtDialer -> Setup (as the account owner) and drag & drop your users to the available seats.
All users can access smrtDialer, but only users that are assigned to seats can run a campaign and make calls.
Here is a video on how to assign users to seats:
Learn more about how you can Assign agents in your campaign >>
Learn more about How to log in to smrtDialer and see campaigns that are ready to be called >>
Learn more about smrtDialer Admin - Get Leads into Campaigns >>
Learn more about smrtDialer Admin - Setting Core Campaign Details >>