Selective realtime location sharing

  I went to dinner last night with a couple of old friends who live on the Upper West Side. As I left my apartment, I thought I might text them to say I was on my way. As I walked to the train station I thought about an app I wished existed.
  I'd like to connect my iPhone with the computer at my host's home, just for the next 1/2 hour, so they could watch my progress. That way if I was going to be ten minutes late they would know as soon as I knew. Obviously this is an opt-in thing on both ends.
  Similarly if I'm going to visit someone who's going to pick me up at a train station. I used to ask people who were visiting me in Berkeley to text me when they got to the MacArthur station on BART. That was approximately when I would have to leave to meet them at the North Berkeley station. This could be automated.
  Is it possible this app already exists?