Toward the end of the discussion I mentioned my situation – the fact that I'm living with my Christian family, and that I'm really worried about how they'll react once I reveal my nonbelief to them. One of them gave me some very good advice: trying to go about it slowly, starting with a vague mention of doubts, will only make it more difficult. They would probably try to get me into some Christian counseling or something. Instead I break it to them all at once, possibly with a one-page written explanation of why I no longer believe.
Before I left they recommended I read books by Dawkins and Hitchens, and specifically Sam Harris' The End of Faith and Letter to a Christian Nation. Overall, the group confirmed my initial impression: they were intelligent, articulate, friendly and supportive. I definitely plan to attend meetings in the future.