To assign more than one user to a portal, the best thing to do is set up a password protected portal instead of adding a new user the traditional way.
From your WordPress dashboard, navigate to Projects -> All Projects.
Select the portal you want to assign multiple users to.
Under Publish -> Visibility, click Password Protected and enter a password to share with your users.
Now simply share the URL for your portal and the password with as many different people as you like! (They won’t need an email address to log in)
Another option for adding multiple users to a project
From within the project, in the Project Information tab, scroll down to find Client Accounts. Click in the field and a list of your users will appear. Click on the name of the user you want to add.
To add another user, click in the field and the list will appear again. You can add unlimited users and they each will access the portal using their own unique login and password. To remove a user, click the “X”.