Someone sent you an invitation to an event as an .ics file, and you wish to add it to your Google calendar

The first part is to import the file. To do this, click the + icon to the right of the “Add a friends calendar” located on the left side of the page. After clicking the icon, from the available options select ‘Import’

Now, the obvious thing to do would be to just to click the button that says “Select file from your computer”, select the .ics file and click import, but if you would do that, chances are that you’ll see the dreaded:

‘Failed to import events: Could not upload your events because you do not have sufficient access on the target calendar..’

The solution for this is to manually edit the .ics file prior to importing it and replace all occurrences of “UID:” with “UID:X” (without the quotes). After doing this and saving the file, proceed with the import and all should be fine.

Learned this from here