A. From the GUI, you can only grant a user Send-on-Behalf rights for another user's mailbox one at a time. For bulk assignments, you need to use the Windows PowerShell Set-Mailbox -GrantSendOnBehalfTo "<user/group>" command, and pipe the list of users' mailboxes for whom you're granting another user Send-on-Behalf rights. For example, to grant the Admin group Send-on-Behalf rights for every mailbox in an organization, use the Get-Mailbox command to get a list of every mailbox, then pipe the list to the Set-Mailbox command, as the following code shows:
You can use the Set-Mailbox command to change other permissions, as well.