Since "I'm not going to ask my friends if they have Bail installed, and presumably Bail needs to know an event on their side and my side is the same event" was my first thought on reading this concept, calendar integration seems practical.

However, I have an issue right following that idea: I rarely ever send calendar invites to social events. In the example on the site, "Drinks after work", that's an event I cannot possibly imagine sending as a calendar invite for, so it's baffling to me how I could Bail it.