We know from Voldemort that Lucius was responsible for the Death Eater march at the QWC. He attended the game with Narcissa and Draco. When Harry, Ron and Hermione flee into the woods, they meet a very relaxed Draco, leaning against a tree, alone. If only Lucius was part of the gang, where was Narcissa? I doubt she would have just gone home and let her son watch as her husband anounced the return of the Death Eaters. Btw, Harry also suspects both of Draco's parents to be part of the Death Eater group, and Draco just smiles.
And: Have you noticed that the only Death Eaters that Voldemort addresses with their first names are Lucius and Bellatrix? Yes, you can assume that those two are maybe the closest to him, but it could have a fully practical reason. It wouldn't be very usefull to call the Lestranges by their last name, as there are three of them. It would only lead to confusion. Lucius follows the same pattern when addressing his fellow DEs, only last names, excpet for the Lestranges. So, perhaps the fact that Voldemort addresses Lucius with his first name means that there are more DEs with the name Malfoy?
And one more thing about the Death Eaters at the QWC (It's really a scene worth re-reading). Don't you find it scary how exactly it matches Snape's worst memory? JKR put quite an emphasis on the fact the one member of the group turned a poor muggle upside down, presenting her knickers, with the same spell James used on Snape. Now, considering the fact that Snape tends to treat people exactly the same way he was treated as a child, it certainly brings up the question where dear Severus was that night. Actually, I find it likely that he was part of the group. Surely Lucius would have asked his "lap dog", no?
Even if that's not the case, the resemblance is scary. It shows that the methods of a 15-year-old James Potter were the same as that of a group of sadistic dark wizards, and also how people tend to watch and laugh rather than help. Except for Lily and Hermione, that is.
Yeay, that's one long post!
