You must have "owner" access or be logged into the email account
1. Right click on Account > Data File Properties
2.Permissions Tab > Add...
3. Find user and Add > OK
4. Select the user and set their Permission Level, Then Press OK
5. Right click on the individual folder > Properties...
6. Go to the Permissions Tab and repeat giving access like step 2-4
You will have to do this to each folder inside of the mailbox.
For the other user, Use this guide to add the account: https://asihelpdesk.fullerton.edu/support/solutions/articles/35235-how-do-i-add-a-secondary-mailbox-once-permissions-have-been-granted-