Adding users to the system and setting their permissions is a two step process:
1. first add the users email and password combo under 'user management' -> 'add user'
2. set their specific permissions per site
Please note that if you do not see either the 'add user' or 'set permissions' window appear, this may be due to a pop-up blocker. Please try turning off your pop-up blocker while you are managing permissions to determine if this resolves the issue.