Hi Sam,
I believe that you have created a Sales professional trial and it shows up in your Dynamics 365 admin center with the instance type 'trial' as shown below:

You can add users to this type of trial by following the steps below:
-Create the users in the Microsoft 365 admin center (https://admin.microsoft.com/Adminportal/Home?source=applauncher#/users)
-Now navigate to the PowerApps admin center (https://admin.powerapps.com/)
-Click on 'Environments' and click on your Sales Professional Trial environment.

-Click on Security and enter the email ID of the user that has been created in Office 365 and click on Add user.

-You will see a message in green displayed stating that the user is being added.

-You can check back in your instance under the list of enabled users by clicking on the gear icon on the top right corner -->advanced settings. This will open the settings page.
-Click on the drop down next to Settings on the navigation bar and select 'Security'.
-Select 'Users.'

-You will find your newly added user in the list. I added a user named Salesuser and I see the user in the list.

-Please assign appropriate security roles to the users and assign 'Sales professional App access' role in addition to other roles. The users will be able to access the instance.
Regards,
Shaina
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