Users are created in Damstra Forms on the web, under Admin, then Users. You must be a Super user with Manage Users permission to see this menu item.
Click the green Create button, choose a user type (Super, Standard or External) and enter their details. Each user must have a unique email address, which they use to log into the app (both on the web and mobile app).
The new user will receive an email with a link to allow them to set a password for their account.
Have a look at this guide for more details on user types and permissions.