How can I subscribe to my Friend's calendar?

How can I subscribe to my Friend's calendar?

You can subscribe to your friend's calendar using the iCal URL of your friend's calendar. Go to Settings > Calendar > Subscribe > Web URL option. Enter the iCal URL  and subscribe to it. This will be the unique URL of the calendar, hence it should not be publicly exposed. 

If your friend is a Zoho Calendar user, you can also subscribe to a Friend's calendar, using their email address. Go to Settings > Calendar > Subscribe > Zoho Calendar users and enter the email address. The public calendars in your friend's email address will be added to your Zoho Calendar.

Refer here to know more about subscribing to Zoho Calendar.