It should be safe in an atheist group to identify oneself as an atheist and speak openly about one's views on religion. As you said, it may not be particularly safe to express one's views on politics, feminism, or other subjects outside of atheism, religion, secularism, humanism, etc. But in the other groups, one could not assume this. The hiking or chess groups mentioned could be filled with people who would condemn someone for being an atheist. They could also have a political slant where they condemn everyone who deviates from it. So one thing the atheist group has going for it is that it should at least offer the opportunity to speak openly about some - though not necessarily all - of these subjects.
My recent post Dismantling Pascal's Wager