The “Check In” screen shows you where some people are, and how long they’re scheduled to be doing whatever they’re currently doing (it doesn’t tell you exactly what that is, only the location). I’m not sure how it decides which people to show there, but it should have all your friends who are busy, I think. Maybe the rest are friends of friends or something?
The challenge though is that sometimes even if you pick an event that ends at the same time a friend’s event ends, they may start up something else immediately, before you can invite them to your own thing. I had a significant other do this to me for about a week in one game.