Do not use an apostrophe to indicate the plural of a symbol like "A", because that would represent possession. Rather express it as "As". But that seems confusing. If it is a group, why not just say "Group A"?

You would not say "players A", so therefore to say A's would not work either I am afraid. Are you referring to a team or to a group of players? You could say "the A team" or "team/group A". If you wanted to then refer to these, you could say "The As" but you would not use an apostrophe here. Tom