Hi,
Which version of Liferay are you using?
The OOB Add user portlet cant be added to the page. For the 5.2.3 version you can may be refer to this plugin created for adding users.
Add New user WizardOr you can create your own custom portlet to achieve the same.
Regards,
Tina Agrawal
Firmi prego dentro per inbandierare questo come inadeguato.