Since I've been an atheist, a few months, I've only openly shared with a few people my "beliefs". I don't think it's something that you can freely share without fear of something bad happening to you because of your beliefs. Do you or have you heard of any atheist stories gone wrong? What happened when you told a person. Well when I told my little sister over the phone she went crazy saying "That's crazy. How can you do that? That's wrong you grew up and church and have seen things." I responded "Oh really,, what have I seen that has proved to me that a God exist?", no answer.
I think there are many Atheist are are afraid of "coming out". I think the more Atheist "come out" the easier it will be for agnostics, and we will notice that more people that we thought are in fact atheist too. So tell everyone!