Simply use PowerShell and the Add-RecipientPermission cmdlet. For example, you can create a CSV file, put the list of users in it, and do this:
Import-CSV blabla.csv | % { Add-RecipientPermission $_.user -Trustee ERP_Report -AccessRights SendAs }
where blabla.csv has a column named User, designating the corresponding mailbox. Make sure to use an unique identifier such as the UPN or email address.
