Just to echo a little of what Jason said, I think unless they really need to know, it's up to you. I mean, if it's someone you'd like to potentially form a relationship with, I guess that would be different. But if they're just friends, it shouldn't matter to them what your biological gender is (in a perfect world..) perhaps you could get a better idea of a possible reaction by bringing up LGBT or more specifically, transcommunity related issues or questions and observe what they say. When speaking to people online while I was still questioning my gender identity, I found it to be quite helpful to get an idea of who it wouldn't be alright with and who wouldn't really care.