I was thinking more along the lines of going over Voldemort's head. He influenced him into sparing Lily. Who's got the power, and who's posing as a couch? *smirks*
Originally Posted by SlytherinTears
I don't believe Snape was a part of the Slug Club, for we are under the impression that he is accepted nowhere.
He gets part of his bitterness from the Marauders, and the other part form not being accepted.
Members of the Slug Club during the Marauder Era may have included Lucius Malfoy (Draco said that he was a favorite of Slughorn back in the day), and with Lucius being a prefect in the Marauders' first year, he would have still been in the Slug Club when Lily was at Hogwarts. Slughorn probably didn't know he was a Death Eater at that point.
Theodore Nott's father may have been in the club as well, as Theodore is around the same age as Harry, therefore his parents would be the same age as well. The same goes for McLaggen's Uncle Tiberius.
For other possibilities, you can check out this link at the HP Lexicon.
I hoped this helped,