One license is equivalent to one account. Since they have separate license, the rest should be invited by the owner of the data file that they need to access. The number of users to be invited will base on the plan type.
These are the steps on how to add a user:
Go to the Company tab and select Set up Users and Password.
Select Set Up Users.
Click on the Add user button.
Fill in the details, and then click Next.
Choose the areas you wanted them to have access, and then click Next.
Confirm the access to the user.
You can add additional user after the transaction, just follow the steps I have provided.