The best way to do this is to create a common user ( firstname.lastname@example.org) together with its mailbox , the userID and all the other things and this can be done :
1- on the Lotus Notes Domino administrator select the administration hub server .
2-on the left side click “people”
3- on the left column click “register”
4-register a new common user ( email@example.com) like you do to all new users
5- afterwords on the ACL of the names of the five users as “persons” and the rights must be “manager”
6-then they all can open this database together with their own mailboxes and send and receive message using a common address ( firstname.lastname@example.org)