It's an android / iOS app that runs in the background, and buzzes you when it detects you are hanging out with a friend. You can then decide to record that "With". The only other function of the app is a newsfeed that shows who you've been with and who your friends have been with.
Here's a screenshot of the prototype: http://i.imgur.com/BGAL4XI.jpg
What does Hacker News think - do you think this could be a cool app?