What is an epistasis group?
What does it mean if gene A is epistatic to gene B? Does this simply mean that gene A masks the phenotype of gene B when expressed? 


Yes.  Note that this does not say anything about mechanism. If A converts 1 to 2, and B converts 2 to 3, then A is epistatic to B, but A is also upstream of B.  In a signaling pathway, say B inhibits A and prevents it from signaling. A mutation in B allows A to signal. A mutation in A means there is no signaling. Mutations in both A and B means there is no signaling: A is epistatic to B, but A is downstream of B.

An epistasis group is a group of genes in the same pathway.  Knocking out any combination of these genes will not result in a more severe phenotype than knocking out the most "sensitive" of these genes (the single mutant with the most severe phenotype).