Shared google calendar question

A friend of mine asked for some advice on sharing a google calendar.
I wanted to help, but could not find a way to share it with an email address for someone he’d never sent an email to.

I can add the email address in the sharing options, and select the access rights, but the “send” button will stay greyed out.

Any ideas?

Is it a regular gmail account or work G-Suite account?

a regular gmail account

You should be able to do it. Instructions here

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The recipient must have a Google Calendar account; if not the calendar must be made public.

Maybe a little off-topic, but Woven might be an interesting app in this regard. It is a calendar with complex scheduling options through links in email or even iMessage.