Hello,
I have a simmilar case (10 people work on one mailbox):
Perhaps it helps here is my setup:
Everyone opens webmail with his one proper mailbox.
There is A Mailbox called "info@domain.com" this one has its Folders shared with a "shared group" with those 10 People inside. And i have added every of the 10 People as a cathegory (colormarkings).
In every of those 10 Peoples account i have setup under "settings" -> "connectivity" -> "SMTP" i have put in the login to "info@domain.com" (with 127.0.0.1 as server address).
(under "settings" -> "signature" you can even define the default signature if you select the "info@domain.com)
This means they have to possebility of selecting this adress as the "from adress". (if you reply on a email in the "info@domain.com" mailbox it will automaticly select
info@domain.com as the sender .
For me this works best as the 10 people "helpdeskagens" can categorise mails in the "info@domain.com" with teyr proper color (this prevends others people from working on the same mail, in addition if there are questions everyone can see who worked on this email).