I think the only reason to believe they were not in it, would be personality. We have some definite facts on this subject.
We know that the Slug club existed both before and after they <PERSON> and <PERSON> were at Hogwarts. <PERSON> himself was part of it, very early on, which would be roughly 50-60 years ago from the HBP. We also know that <PERSON> (captain of the Holyhead Harpies), was also involved in the Slug Club. <PERSON> was born in 1968, while <PERSON> and <PERSON> were born in 1960.
<PERSON> Wikia Entry
So, the question is whether or not <PERSON>, <PERSON> and <PERSON> would have been invited. <PERSON> says that <PERSON> was one of his "all-time favorite students", and he is courteous enough to overlook her being muggleborn because she was so talented. <PERSON>, a wealthy descendant of the <PERSON> (not to mention talented student/quidditch star) would have likely garnered an invitation. 899
<PERSON>, a Pureblood from an old and rich wizarding family (also a talented student), also was probably on <PERSON>'s list.
When it comes down to personality it is less clear. <PERSON> tries as much as possible to distance himself from his family, so he may have distanced himself from <PERSON>. <PERSON> may have even decided not to have him be part of it, based on his attitude. (Think about <PERSON>, who admits his father doesn't get along well with his famous uncle, and is never seen in the Slug Club again). After seeing that <PERSON> is unwilling to use his family as a leg up, he may have been passed over for the club, despite his talent. And, based on their friendship, I doubt either would go to <PERSON>'s parties without the other, they were somewhat of a package.
We know that <PERSON>, gave <PERSON> cheeky answers when told she should have been in Slytherin, but I doubt she would have been quite so rude as to decline the occasional party. I am willing to believe that should would have been part of the Slug Club, but there is no proof of that in the books.
And, of course, people like <PERSON> and <PERSON> would have been passed over completely, <PERSON> has no connections and <PERSON> is a useless weenie at best. 899