Say you are Eventbrite....

By collecting data on which event caused them to register an account (and maybe a short questionnaire during the onboarding process), you can send them targeted emails with similar events in their area.

For instance, if they registered to Eventbrite in order to attend a fiction-writing workshop, you could set a weekly job to email them about other creative writing workshops in their area, or maybe alert them if an author's book tour is coming through the area.

It all comes down to the quality of data you're collecting. And, at scale, you'd also need a system smart enough to match people and events correctly. (I'm guessing that Eventbrite does this through tagging.)

edit: To re-engage event organizers, run a special where you'll help promote their next event via some social media ads. If you don't have that kind of money, you'll need to make sure to capture them on the first go-around: make it easy for them to set up, promote, and get RSVPs for an event. Other than that, you might just need to send cold emails to inactive users and hope they reconvert.

For event organisers or attendees?