How do I prevent accidental 'Invitations' in Calendar app and delete old, unrelated events?

In Calendar, I scheduled a new event, for a meeting at 10AM. I entered the event name and the time. No detail.

Shortly, I received an email from a friend, asking if I had "meant to send this" and in her email I could see my event name, a message saying the event was cancelled, and when I scrolled down, a hundred or more names. Some of the named individuals are deceased. I also received many "could not deliver" type messages from GMAIL, and it looks like it was attempted several times (several messages per person). Several friends reported receiving "strange" messages, repeatedly. Was I hacked? A closer look at the event I created showed that somehow, a list of

"invitees" had been created, and the list was headed "Microsoft Teams"... searching Microsoft Teams, I found several 6 year old events that had been created by someone else, and I had been "invited". Those events had the same title as the one I had chosen for my own new event.

I have been able to delete one of the old events I found, but that option does not appear on the others. What happened? I know now to inspect an event I write to make sure other materials haven't been added, but how to prevent this?

[Re-Titled by Moderator]

Original Title: Accidental "invitations" in Calendar