If you wish to apply changes to the GrantSendOnBehalfTo remember (as you will find out at some point I am sure) that the native command will replace all the permissions in that group, first check who is in there with this:
Get-Mailbox Test.Mailbox@pokebearswithstick.com | fl Select GrantSendOnBehalfTo
This will then return the object ID values for the users in the permission like this:
If you then run this command to add a user, this command is run in replace mode:
Set-Mailbox Test.Mailbox@pokebearswithstick.com -GrantSendOnBehalfTo bob.bear@pokebearswithsticks.com
That command will remove any people currently set to that group and replace the contents of that permission with the user you have just specified
Correctly adding Users
If you wish to add a user you need to use this command:
Set-Mailbox Test.Mailbox@pokebearswithstick.com -GrantSendOnBehalfTo @{Add="bob.bear@pokebearswithsticks.com"}
Correctly removing Users
Set-Mailbox Test.Mailbox@pokebearswithstick.com -GrantSendOnBehalfTo @{Remove="bob.bear@pokebearswithsticks.com"}
Correctly removing all objects
Set-Mailbox Test.Mailbox@pokebearswithstick.com -GrantSendOnBehalfTo $null