A calendar invite is the most reliable reminder there is. It sits in the person's calendar and pops up before the event. That's why people use it for webinars, classes, launches and meetings. It only works when the people you invite actually want to come, though: unwanted invites get reported quickly, and both Google and Apple act on that.
Google Calendar limits
- Google limits how many invitations one user can send to people outside their own domain in a short time. For paid Workspace accounts it states the figure as 10,000 external invites in a short period. Free and new accounts get lower, unpublished limits.
- The "Email guests" button has its own cap of about 2,000 emails to external guests a day.
- When you hit a limit, Google pauses that action for a few hours, sometimes longer. Its own advice is to wait, or to split the work across several users.
iCloud Calendar
iCloud invitations go out through iCloud Mail, so the iCloud Mail limits apply: about 1,000 messages and 1,000 recipients a day, and no more than 500 recipients per message.
Tips for big invite lists
- One event, many guests is lighter than many separate events.
- Spread large lists over several accounts and several hours, rather than sending everything at once.
- Write a clear title and description: what it is, who's hosting, and how to join. Vague invites get reported.
- Only invite people who signed up or expect it. This protects your accounts more than any setting does.
Doing it with ZeroPhantom
Google Calendar Sender and iCloud Calendar Sender send invitations to your list from several accounts, with a daily cap and pauses per account, so you stay inside the limits above. Both have a free 3-day trial.