When I share a calendar to another person, he can see it in the SOGo Webinterface. When I then add the account via CalDAV to my iPhone or any other device, I can only see the calendars of the account itself, not the shared one.
SOGo could also list shared calendars on the base caldav URL, just like it lists all your own. However I also just tested this and this feature is not there apparently. Maybe you can ask SOGo upstream about that.
I asked in the SOGo Mail list. It's hidden. You have to enter you calendar account settings and somewhere, you'll find delegations. There you can enable you shared calendars. Kinda hidden and different for every calendar app.