Sorry for the confusion, but we are not sure what exactly you need to achieve. Do you need to add new users as members to multiple shared mailboxes? Or, maybe, you need to grant specific permissions over the mailboxes? For us to help you with a solution, please, describe the desired workflow in all the possible details with live examples and screenshots. If you cannot post screenshots or any other details here, please, send them to our support email (firstname.lastname@example.org).