How compatible are the beliefs of atheists and Buddhism? I looked around and found a couple of questions and answers on the site that discuss atheism in Buddhism, What is the difference between learning the Buddhist way and simply learning from life? &amp; Do Buddhists believe in gods? it looks like belief in a supreme being may not be a requirement for being a Buddhist.  Strictly speaking it is possible for a person to identify as an agnostic or atheist and also identify with any mono or polytheistic church.  Though most (all?) of those actual religions do not support disbelief in their God(s), so they are not actually compatible. 

Are the concepts of agnostic or atheist believers compatible with Buddhism? Does the entire belief system or specific sects require the belief in any or a specific supreme being?
I would say compatible. There is a significant Secular Buddhism movement based on agnostic, humanist and skeptical values. Stephen Batchelor is a notable proponent of this school for instance in his book Buddhism without Beliefs.

I don't think you need to go the whole way to Secular Buddhism though to appreciate the Buddha's pragmatic approach to the Dharma. In the Cula-Malunkyovada sutta he refuses to answer a whole list of metaphysical questions regarding them as a distraction for instance


  "Malunkyaputta, did I ever say to you, '[...] that 'The cosmos is
  eternal,' or 'The cosmos is not eternal,' or 'The cosmos is finite,'
  or 'The cosmos is infinite,' or 'The soul &amp; the body are the same,' or
  'The soul is one thing and the body another,' or 'After death a
  Tathagata exists,' or 'After death a Tathagata does not exist,' or
  'After death a Tathagata both exists &amp; does not exist,' or 'After
  death a Tathagata neither exists nor does not exist'?"
  
  "No, lord."


Also in the arrow sutta he insists that the thing to do is engage in the practice and not to be put off by useless questioning. To paraphase if you are struck by an arrow don't concern yourself about who made it, where it came from, what direction it was fired from - just pull it out. So if you are experiencing suffering, don't worry about rebirth, the gods, ritual etc.. just pull the arrow of suffering out. Engage with the Dharma as soon as you can.