Are KS members all atheists?
Not at all. Some Humanistic Jews are atheists, some are agnostics, and some are deists who believe that there is a power which created the universe and is expressed in natural laws. Many Humanistic Jews may feel an awe and reverence for the wonders of nature or the human spirit -- a form of spirituality -- but don't envision the kind of God that one might address in prayer.
Of course, anyone may become a member. If someone joins us who believes in a personal God, we respect that belief, and anyone's personal beliefs, so long as they aren't damaging to others [beliefs such as in racial inferiority, etc.] and so long as the person doesn't expect us to change to conform to his/her beliefs..