I recently wrote that the Group Policy Management Console Sample Scripts could be used to grant the AGPM service account rights to all existing GPO’s.
Another way to do this if your running Windows 2008 R2, is to use the Group Policy cmdlets in windows PowerShell.
First you must import the Group Policy module with the command import-module grouppolicy
And then you will be able to use the cmdlet to grant the service account access, in this example the security group AGPM-Service will be granted full access.
Set-GPPermissions -All -TargetName “AGPM-Service” -TargetType Group -PermissionLevel GpoEditDeleteModifySecurity
For a complete list of Group policy cmdlets see Group Policy Cmdlets in Windows PowerShell.