I didn't tell anyone at first, not even my husband, because we were not in a good place in our marriage at the time and I knew he would scoff at the changes I was making with remarks along the lines of, "You can't ever stick to these big changes you say you're going to make." So I kept it to myself.
He noticed within a few weeks that I was changing the way I ate--because the major change I was making was to not eat sugar. And that was a huge change for me. We began talking about it, about a month in, and I urged him to try it out, too. (Which he did--a huge step for him, as he loved his Coke--and thus began his weight/health change.)
My coworkers noticed after a few months. I think I got my first comment about losing weight from one of them after I'd lost about 40 pounds. For the most part, they have not said much, but they have been very encouraging and supportive and understanding. They don't try to press birthday cake on me, etc. I think they all think I'm a little crazy, but it's a kind of crazy they respect
My mom and other relatives of course have noticed and have been very supportive. Close friends as well.
In fact, I have not heard one negative thing from anyone, this whole time. I wonder if that's perhaps about where I live--northern California--where everyone is pretty health-conscious, even if they're not actively doing something about it.
So I guess my answer is, I kept it to myself at the beginning when I was unsure whether I'd be able to stick to it. But once I knew this was really how I would live my life, I relaxed. I still don't go around "informing" people, but if it comes up, I'm open and honest about how I have lost weight.