Share a single calendar with iCloud
If on mac:
- Open up calendar app.
- Right click on the specific calendar you want to share with someone.
- Click on "Share Calendar..."
- Write the name of iCloud user you want to share that calendar with.
If on ios:
- Open up calendar app.
- Find the calendar you want to share.
- Press the "i" icon aligned at the right of each calendar.
- Click "Add Person" and write your friends iCloud adress, name, whatever..
I gotta mention it mainly works on iCloud calendars, not for lets say "Google" account's calendar.
I hope it helps, feel free to comment here if you got any more question.