If the user is getting the error "username not found," it means they are NOT set up as a user. Just go to Manage Users/Add user and add them. They will get the automated email to set their password.
If the user is getting the error "username or password is incorrect", then it means they are entering their password incorrectly. Passwords are exactly the same on the web and on the app. Have them re-type it OR they can always reset their password.