Users can be invited by adding them in the role specific groups under Enterprise Dashborad. Once you have added a new user in the role specific group, they should receive an email with an invitation to Adobe Target Standard. Once they have accepted the invitation you can decide their permission level under Setup > Users within Target Standard.
For Target Classic the Admin has to add the user in the interface and then provide the users with the password generated for them. See the below docs for more on this: