Don't know if I missed it anywhere the solution my question.

So here is the story. I don't want to make my calendar publicly
available but want my friends to add events into it. These friends
don't have Gmail account. so when I 'share' this calendar with them,
it pops up the message if you want to invite them to use Google
Calendar, if I say don't invite then they don't get email but if I
invite them Google asks them to create Gmail account first.

How do I manage to give my friends access to my Gcal without making
them open a Gmail account?