You can make use of Active Directory. As I understand it, Active Directory is a directory services database, and LDAP is one of the protocols you can use to talk to it.
Configuring LDAP support for adding users includes Active Directory. This Adobe Blog post is an excellent overview of how it's configured:
http://blogs.adobe.com/techcomm/2011/02/enhanced-config-mgr.html
At the present time, MySQL is not a supported database for RoboHelp. Only the ones listed in this knowledge base article:
Recommended OS/database combinations | RoboHelp Server 9
Thanks
John Daigle
Adobe Certified RoboHelp and Captivate Instructor
Evergreen, Colorado