The more I look at the list of qualities associated with the Slug Club the more I think that Harry's parents - and probably some of their friends - must have been part of it.
[Slughorn] also likes the company of the famous, the successful and the powerful. He enjoys the feeling that he influences these people. He has never wanted to occupy the throne himself; he prefers the back seat – more room to spread out, you see. He used to handpick favourites at Hogwarts, sometimes for their ambition or their brains, sometimes for their charm or their talent, and he had an uncanny knack for choosing those who would go on to become outstanding in their various fields. Horace formed a kind of club of his favourites with himself at the centre, making introductions, forging useful contacts between members, and always reaping some kind of benefit in return, whether a free box of his favourite crystallised pineapple or the chance to recommend the next junior member of the Goblin Liaison Office."
(Half-Blood Prince, Chapter 4, Horace Slughorn).
- James Potter came from a super-rich family with a knack for inventing the next great bestseller in potions.
- Sirius came from an ancient pureblood family with excellent connections.
- Lily didn't come from a well-known wizarding family but was basically his favourite student.
None of these three were stupid. In fact, all of them are described as being extremely bright. If it hadn't have been for the war they would no doubt have had glittering careers before them.
We know that Slughorn was at Hogwarts at the time. Did he recruit James, Lily or Sirius to the Slug Club? Why/why not?