how would an "atheist group" of two people actually work? I think we should be told.

Lets start with the definition of the word "group."

Definitions of group on the Web:

    * any number of entities (members) considered as a unit
    * arrange into a group or groups; "Can you group these shapes together?"
    * (chemistry) two or more atoms bound together as a single unit and forming part of a molecule
    * a set that is closed, associative, has an identity element and every element has an inverse

Say, two Athiests, say Smokey and Smarty Shaw, regularly get together to share believer jokes over warm beer and call themselves something suitably modest such as "the true arbiters of the one true truth and representatives of all non-aligned Atheists" That would be a group, a "unit" if you will, of two.