I have not been open about it until very recently due to having to be and taking the right meds (so I'm not paranoid). I generally start with just being open about my moods and sometimes advancing to the dx. I won't go out of my way to say it but I'm trying to become fearless about telling people. Why? Because if I ever want the stigma against MI to change I need to talk about it too. I can't just hide as much as I want to. Not easy but a work in progress

Edit: to answer your question I think the first person I told was a friend who's followed my progress for a while. She was supportive as in I was getting treatment I needed... not everyone has been so supportive.