Unlike for a default environment for a new trial environment, your Office 365 users won’t get the access automatically and you can’t manage this access via Microsoft Office 365 Admin portal. Or even via model-driven UI for your newly set up environment. You have to add users one by one using the steps below.
From PowerApps Admin centre (and I mean NOT THIS ONE!
https://admin.powerplatform.microsoft.com/) this one: https://admin.powerapps.com/environments
Click on the environment you would like to set up users on. Go to the Security tab.
Type user login for Step 1 to add a user. Click Add User.
As a result, your user will appear under Enabled Users on your environment.
Click on the Assign Security Roles button.
I forgot … You need a System Customizer role to be able to create your own entities.
More info is here: https://docs.microsoft.com/en-us/ai-builder/security