When creating an Exchange Server 2013 DAG you may receive an error that the Exchange Trusted Subsystem is not a member of the local Administrators group on specified witness server.
Configuring Database Copies in an Exchange Server 2013 Database Availability Group
How to add a database copy to a Database Availability Group member in Exchange Server 2013.
Installing an Exchange Server 2013 Database Availability Group
Step by step guide to installing an Exchange Server 2013 Database Availability Group