To grant access to a resource mailbox, you must use the Exchange management shell.
Assuming you already have the mailbox created, and it is called “ResourceMailboxName”, here is the command you should run:
Add-MailboxPermission -AccessRights FullAccess -Identity ResourceMailboxName -User Username
If you need to grant a different level of permission, use a different option with the ‘AccessRights’ switch. The other options are:
See these pages for more information on resource mailboxes: