How and why did she get in?

I'm brand new to Zoom.  I hosted a meeting yesterday, and today I got an email stating that someone I don't know joined my meeting (which, again, was yesterday).  

How did she join? I'm guessing someone forwarded her the invite, giving her the meeting info?  And why would she join an old meeting?  

And, lastly, what do I do about it?  I deleted yesterday's meeting.  Is that enough?