This feature is only available to the administrator of your account.
The administrator can access the "User management" page through the navigation menu on the left hand side.
On the overview page, all users with access to your company account will be listed. If your account is connected with a partner account (e.g. external law firm), you will also see the partner users.
In the upper right corner, it shows how many users you can add to your account. Partner users do not count against this number.