Reading a Google calendar meant finding its secret iCal address and pasting it in — a link that is awkward to locate, easy to leak, and impossible to revoke without regenerating it. You can now connect the account itself and pick which of its calendars Keeper.sh reads.
Signing in also means Keeper.sh knows what it is looking at. It lists the calendars on the account by name, so choosing between a personal calendar and one shared with you is a matter of ticking the right box rather than working out which link belongs to which.
A calendar can now be both a source and a destination without feeding itself. Keeper.sh recognises the events it created and skips them on the way back in, so pointing two accounts at each other produces one copy in each direction instead of a loop that grows on every run.
Read more about everything Keeper.sh does.