Originally Posted by X-Maniac
When Stryker was chatting to Kelly and the President in X2, there was mention of the mutants picked up at Liberty Island, so the absence was covered in as much as it was implied they were all in captivity.
Of course we saw Magneto in his plastic prison and in a never-filmed sequence from the X2 script, Toad was at Stryker's dam where he had a fight with Nightcrawler.
Well there you go. A simple mention like that could explain what happened to the Hellfire Club members. As for Banshee and Havok, their absence definitely needs to be explained imo. Regardless of how popular they are they still played a role in First Class and people will remember them.