Let users of an inbox be able to see what other users have access to this inbox.

Users of a shared inbox need to know who they share the inbox with.

It is indispensible for cooperation and communication that users of shared inboxes know who else has the information they have. Most of the users are not administrators. Administrators may change the access rights and (other) users have a strong interest to always know, who has access currently.

There are many possible ways of implementation. One way would be to use the three vertical dots on the right side of the shared inbox name on the sidebar (when hovering over) and add "Show users" or similar.