When I quit the seminary and left the church I tried to slip as quietly out the back door as I could. At this point my theology professors from college, many of my college and seminary friends, and most of my extended family know I'm an atheist. I officially resigned my membership with the SDA church almost two years ago. Now I'm an atheist blogger and I've talked about the SDA church on the Chariots of Iron podcast a few times.
I'm as out of an atheist as I can be, both with those inside the SDA church and everybody else.
I was raised in SDA family. I began to doubt the idea of God after reading Cosmos by Carl Sagan at age 15. Took me about 20 years to finally become an atheist. Just shows how hard it is to undo things you are indoctrinated as a child sometimes.
I think I've lost track of how out I am with some people. A lot of my extended family has no idea. My brother knows everything. My parents probably still think I'm agnostic (I just haven't felt the need to update them). All my good friends know, and many of them are quite supportive, even while still being fully Christian. (I don't know how exactly that works for them, but I'll take it)
As far as generally "out", I don't lie to anyone, but I don't make a big deal about it either. If it comes up, I'll straight out tell it like it is.
I also am very out, even though most of my family members are still SDA. I'm sure they're still praying for me.
I had the fortune (I guess) of getting away from Adventism before becoming atheist so I didn't really have to deal with any former Adventist communities, other than my immediate family. I have mostly stayed in the closet about my atheism, but do post lots of science and even some humanism-related posts on my FB. My guess is they probably have an idea that I'm no longer Adventist but its not something anyone really asks me about and I'm okay with that. Sometimes I do wish I could be more open with my family about it, though. Shedding cognitive dissonance is such freedom.