Most of the shared calendars I have seen allow you to ''see'' other's calendars. There are some special cases where a room or car has its own calendar so staff can book it, so in principle, this provides evidence that it's possible to operate a calendar that multiple users can read and write. However the examples I am thinking of are in a closed Office365/Exchange/Outlook system, and probably use services on the server to support this mode of use. Typically, a Google or Yahoo calendar is personal. Only its owner can change it. You can invite others to an event and they will be offered some way to add an invitation to their own calendar. I dare say you could create an account at Google or Yahoo (etc) and share its credentials between multiple users. They could each login to read and write to that account's calendar. I don't know how it would cope with different users making simultaneous or concurrent efforts to access it. Multi-user access to a shared resource needs special techniques; typically the user accessing it has, temporarily, sole and exclusive access to the shared resource, so there can be no concurrent changes by other users. If an attempted change fails, (for example, due to a communications failure) the system has to be able to roll back to the state it was in before the attempt. I really don't know if any of these calendars designed for personal use have this level of technical sophistication.