First there's this, from the Help page at https://support.google.com/calendar/answer/37082?hl=en
Emphasis is mine.
You can share the main calendar for your account, or another calendar you created.
- On your computer, open Google Calendar. You can't share calendars from the Google Calendar app.
- On the left, find the "My calendars" section. You might need to click it to expand it.
- Hover over the calendar you want to share, click More More and then Settings and sharing.
- To share with individuals: Under "Share with specific people," add the person or the email address of the person you want to share with.
- To change wider sharing settings: Under "Access permissions," choose an option in the drop-down menu. Learn more about these options below.
- If someone isn't already added, click Add people.
- Add people and click Send.
If you shared your calendar with an individual email address, they'll see your calendar in their "Other calendars" list. If you shared your calendar with an email group , they'll see the calendar in their "Other calendars" list once they click on the link in the email invitation from Google Calendar.
Then there's this: https://support.google.com/calendar/answer/37100?co=GENIE.Platform%3DAndroid&hl=en
Add using Email Address
Ask to see a calendar
- On your computer, open Google Calendar. You can only edit Google Calendar sharing settings from a computer, not the app.
- On the left side, click Other calendars.
- In the Add a friend's calendar or Add a coworker's calendar box, enter the other person's email address.
- Press Enter.
- Depending on how their calendar is shared, one of the following will happen:
- If the calendar isn't shared with you, you'll see a form to request access.
- If the calendar is already shared with you, their calendar will be added on the left side under "Other calendars."
- If the other person doesn't have a Google Calendar, you'll see the option to invite them to use Google Calendar.
Approval by the other person
If their calendar isn't shared with you, the other person will get an email requesting access.
To accept your request, the other person should:
- On your computer, open the email you sent requesting access.
- Click the link in the email.
- In the Settings page that opens, choose a permission setting and select Add Person.
- Click Save.
After the other person has accepted your request, their calendar will appear on the left side under "Other calendars." If you don't see it, try refreshing the page.