Hi, seems interesting, but I was thinking what separates you from the default Facebook events or when a user needs to post an update about the time he is coming?
I think facebook events is more catered towards events i.e weddings, parties etc and not the weekly casual hangouts we do with our friends. Moreover I haven't met a lot of users who actually use it despite it being around for a long time.
About your question, the user doesn't have to enter any information for ETA. The app calculates that based on the hangout destination and your current location.
We have worked on making the tracking part as efficient as possible(code optimizations). Also the tracking will be on for a very limited time 45 mins to 1 hour(around decided time to meet) and the interval for location updates will not be very small hence helping in battery preservation.
About your question, the user doesn't have to enter any information for ETA. The app calculates that based on the hangout destination and your current location.