Search-Mailbox unfortunately doesn't work against Group mailboxes. eDiscovery does however, so it might also be an option - run a search against the Group mailbox and export the results to PST file, then import them into the shared mailbox.
Search-Mailbox unfortunately doesn't work against Group mailboxes. eDiscovery does however, so it might also be an option - run a search against the Group mailbox and export the results to PST file, then import them into the shared mailbox.