I have an Exchange 2010 server on prem – needed to grant send on behalf to a user for a distribution list. In this case the distribution list is called “Events”. However, every time I would change the permissions I noticed that the previous entries were removed.

List Group Current Permissions
Get-DistributionGroup "Events" | fl name,grant*

Replace Granted Users List
Set-DistributionGroup "Events" -GrantSendOnBehalfTo "Some User", "Another User", "Third User"

Add Additional Users
Set-DistributionGroup "Events" -GrantSendOnBehalfTo @{add="New Hire", "Manager Name"}

Remote Users
Set-DistributionGroup "Events" -GrantSendOnBehalfTo @{remove="Recently Fired", "Another User"}