If you're a developer like me, for sure you will prefer to add new users via terminal via Magento Cloud CLI, not just add users, but you can edit, remove, list and edit the permissions via terminal. But in case you want to go to the visual way, I've written the post How to add new users to the Magento Cloud panel.
Specifically to control you users you have this available commands.
Add a user to the project
Delete a user
List project users
magento-cloud user:list [users]
View or change the user role
The following examples use the CLI to add a user, configure roles, and modify project assignments and assigned user roles.
To add a user and assign roles:
Use the CLI to add the user.
Follow the prompts to specify the user email address, set the project and environment roles, and add the user:
Enter the user's email address: email@example.com Email address: firstname.lastname@example.org The user's project role can be 'viewer' ('v') or 'admin' ('a'). Project role [V/a]: a The user's environment-level roles can be 'viewer', 'contributor', or 'admin'. development environment role [V/c/a]: c Summary: Email address: email@example.com Project role: contributor Adding users can result in additional charges. Are you sure you want to add this user? [Y/n] Adding the user to the project
This operation triggers the Cloud build and deploy process, which takes your site offline until deployment completes. For production environments, we recommend completing this operation during off-peak hours to prevent service disruptions.
After you add the user, Magento sends an email to the specified address with instructions for accessing the Magento Commerce Cloud project.
The following example changes the environment-level role that is assigned to a user:
magento-cloud user:role firstname.lastname@example.org --level environment --environment development --role admin
magento-cloud list command displays all the
magento-cloud CLI commands.