The firewall can use RADIUS and LDAP servers to authenticate users from remote sources.
User Manager Support contains information on which areas of the firewall support these servers
To add a new server:
Navigate to System > User Manager, Authentication Servers tab
To edit an existing server, click next to its entry on the same page.
Each type of authentication server is covered in the following documents
Hangouts Archive to view the August 2015 Hangout on RADIUS and LDAP.