How can I add an event on someone else's Google Calendar?

In order to make changes or addition to anyone’s calendar, he/she should have shared that calendar with you with permission to edit. Or you can create a calendar and share it across people. Follow the below step :

1. Open your Google Calendar.

2. Under ‘My Calendar’ tap the three dots near the calendar that you want to share.

3. Go to ‘Settings and sharing’.

4. Under ‘Access permission’ slot, blue tick the ‘Make available to public’ option.

5. Click on the ‘Shareable link’ and copy the URL. Share wherever you want