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:

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