Work Add-Ons are licensed by user tiers. One way to limit the number of users in Dynamics 365 is to use Office 365 security Groups.
You must have global administrator rights in the Office 365 Portal- https://portal.office.com
In the Admin Application:
- On the home page, Groups
- Click on Add a Group
- Select the Type as Security Group
- And add Members to this Group.
- Navigate to Admin Centers->Dynamics 365
- Identify the Dynamics CRM instance you want to work with and Click edit
- Select the Security Group in the Security Settings.
This will now enable only the users in the Security group to have access to the Dynamics 365 instance.