I wouldn't go en femme and just surprise them. You know your family best, but in every group there are likely to be people who don't take it well.
I would suggest giving them some warning. An email a week ahead of time gives any who are over-the-top offended a chance to cancel their plans, and gives those who are supportive time to prepare a welcome for you. (Gotta think positive!)
You could wear a binder and go in guy mode, but that is just postponing the inevitable.
Surprising people with this news by just appearing in person seems disrespectful to me. There are several groups that I will be coming out to. In all cases, I will be giving advance notice to the leaders, then verbal notice (in guy mode) to the members, before appearing as myself.