The process for adding and setting the access rights for a user requires the following 2 steps to be completed:-
First the basic detail of the user, user group and language
The 2nd step is setting up the login access if a single company licence you will see only one row, but you must check the 'Can log in' box for the person to be able to access.
From here you can also set the person up as a performer ie they can work on maintenance tasks, notification and if you are using departments to restrict what is seen its also setup here.
You can access and change these settings any time from the Settings > Users > Users and double click on the person you wish to edit. You see three tabs and the login access is on the 'Companies' as per the above picture.