To assign permissions to Active Directory users run Xopero Management Center on the domain controller. Then click on Active Directory users mapping button marked on a screenshot below.
This icon can be greyed out, if you don't have enough rights. It's the best to login to host as domain administrator.
Active Directory - Users settings window will appear, it contains a list of available domain controller users. At this moment you need to define, to which group the user(whose account will be performed backup) will be assigned.
Available groups are Endpoint and additionally, depends on bought license version - Server. Endpoint group enables members of it local data and mailbox from MS Outlook. While Server expands those possibilities for advanced copies.
Remember to install the Management Center on a different host than the Xopero server. Otherwise, the endpoints will not be able to log in.