When someone wants Sent Items to show in the shared mailbox (as well as their own): Use Exchange PowerShell module to connect to Exchange online (here are the instructions for downloading this), then enter the following commands (the shared mailbox name is conventionguest@ufcw.org here as an example:)

If a user is not able to search for specific emails in the mailboxes that they have access to. Follow the steps shown in the below link.  Outlook 2016 - Search Function Not Working on Shared Mailboxes (naturalnetworks.com)