Hi @Anonymous,
Which user do you want to add into your environment? An external user or the member user in your Org?
Currently, the Security model has been changed in PowerApps and now is managed by Dynamics 365 user management center.
If you want to add an external user (not in your Org) into your environment, I afraid that there is no way to achieve your needs in PowerApps.
If you want to add a member user (who are not in the list of users in this environment) of your Org into this environment, you need to type the email address of the user, in your organization, then click "Add user". You need to Wait for a few minutes to check if the user is available in the list of users in the environment.
More details about configuring environment security, please check the following article:
https://docs.microsoft.com/en-us/powerapps/administrator/database-security
Please click the "list of users" link button to check if the user you want to add into this environment is existed within it.
If the user is still not existed in your environment, please check if the account of the user (you added) has been removed/disabled from your Organization.
Best regards,
Kris
