If the witness server you specify isn’t an Exchange 2013 Preview or Exchange 2010 server, you must add the Exchange Trusted Subsystem universal security group to the local Administrators group on the witness server. These security permissions are necessary to ensure that Exchange can create a directory and share on the witness server as needed. If the proper permissions aren’t configured, the following error is returned:
Error: An error occurred during discovery of the database availability group topology. Error: An error occurred while attempting a cluster operation. Error: Cluster API “AddClusterNode() (MaxPercentage=12) failed with 0x80070005. Error: Access is denied.”
Update: you must add the Exchange Trusted Subsystem universal security group to the local Administrators group on the source database server.