How can many users use same e-mail address?

How can many users use same e-mail address?

Hello,

We have non profit NGO and 5 people are using 1 email address.

How can 5 people enter the same e-mail, and each each of them has unique name and signature?
Basically I want that when you click to compose mail you can choose who sends this email and signature will change correspondingly (different name and suriname, different signature but same mail@)

I don't want to add aliases because if I add johndoe@mail.com, and that person leaves our organization then we won't receive mail if someone send mail to johndoe@mail.com and we deteled that e-mail address.