If someone tests positive, they would ideally notify us and tell us what event they attended. Then we would check all devices that checked-in to that event and send a notification everyone who attended that event.
Otherwise, we could use the distance idea as you suggested but I fear that will introduce privacy issues.
I believe the tricks introduced by @Steve should be a point of visit for you to implement mechanism to capture the user data, such as user email address, which venue they visit, what time etc.
Once the user interact your app, the first thing they need to do is to fill the form on demands of yours to captures those set of data.
Then consume that table to send the notification as you want.
Yes, there is privacy issue as always, which is not an issue of Appsheet, but you should be deal with it by asking for “concensus” how you deal with private info over the app. For instance, before they go deeper into your app, app pose a question for concensus, and you keep record that concent in the separate table etc.
Bunch of works around which make impossile to possible.
With permission of app interactors, you can caputure lat/long data where the form was filled, then user that value to dynamically calculate the distance with distance() expression from a target location. Then do valid test if it is within short distance or long to decide if you submit the notification etc in combination of REPORT features to fire those valid test on schedule + automatically submit the notification etc.