How to synchronize event calendars from different services?

Question

Google

Users registered in different Google, Yandex, Mail.ru systems have calendars with events. You need an online or offline service that synchronizes event data at one point. When specifying a participant in an event in one calendar, the event was created for the invited user who uses a calendar from another company. When adding an event to the calendar, it was mirrored to the calendar in another service.