There is no problem, to share calendar between users (and not only calendar, but also emailboxes). Example of calendar sharing see below:
(Set share over WebMail : https://kc.metransrail.eu)

Add user, for which one you want to set sharing of calendar :

After selecting user, you can select access permition to your calendar :

Newly shared calendar will be automaticaly displayed in Webmail.There is no need of adding of shared calendar.In MS Outlook, you can open shared calendar by adding of emailbox in account settings.