Currently humhub doesn't have option for creating users from admin panel. You can create users manually this way:

Add email on user registration page.

Than open your database with phpmyadmin and from user_invite table copy token for email you entered.

Example token - 54131c28134b61209974487

add token to the end of url like this:

www.yoursite.com/index.php?r=user/auth/createAccount/token/54131c28134b61209974487

Visit url and proceed with registration...