We told people as we saw them. In the end, you know your family, we don't. But in all honesty - I don't know that this really needs to be a "get everyone together". People will be happy and excited for you, but do they really need to set aside a night for this announcement? The only exception is the parents - if you feel your parents will be upset to know that the other set found out first (even if just by minutes), then maybe invite both sets of parents over and tell them at the same time.
getting together for mother's day dinner, we had my parents and his parents over. on the inside of the mother's day cards for our mom's was a picture of the ultrasound. we called our siblings, grandparents, close friends, some aunts/uncles. posted several days later on facebook.
We haven't told anyone yet with this pregnancy but with DD we told my parents and his separately. We gave them each a picture frame and said it was for photos of their first grandchild.