When you want to create a user portal, you should enable the user portal, and then, the "User Portal" button will be appeared on the top of the page.
Click on the "New Table" and you can see the user's table on the page.
There are three ways to add the new users to this portal:
The user can sign up from the app's public link and it will be added to the users' table automatically.
You can add the users manually. Create a new user table, click on the +New Table, select +New row, and insert the users' data manually
Also, you can upload/import your users' data; Click on +Import data and upload the users' data.
There is also a template to create the file. Download it, and fill out the downloaded file with your data. Then upload it to your portal.
Connect to your Bases: If you currently have your users' data available, select it from the search bar and connect it to your portal.