Let’s start here: in EGroupware a user has only a single calendar, so no way to create additional ones.
But you can create a functional user, for a further calendar and set the ACL to allow e.g. yourself all rights on that calendar, and others e.g. read rights only.
The above functional user, can be used to archive that.
Group calendars (selecting a group in calendar) is only a view, which shows the calendars of all the members of that group. You can also invite a group, which “places” the invitation in each members calendar, which he can accept or reject then.
EGroupware mostly uses categories, where are systems use additional calendars.
Hope that explains the concept a bit …
Ralf