Alright, I'll have the two sisters, [i]and[/i] Max (if you re-post that edited Slytherin version of his sheet that you had up for a bit). Max will have been invited (by Emma, or whomever), and then, when Grant heard about what they were doing, he will have pestered Max into bringing him along, against the wishes of most of the girls.

:) Does that work? It gives Grant a more believable excuse for being there (as Max's friend).