Re: 'X-men: DOFP' Official CAST Thread (announcements and discussion) - Part 1
It's been 10 years, Emma joined Magneto and possibly had a fall out and therefore took Azazel, Angel, Riptide with her to form another Hellfire club. Magneto then recruited new mutants.
Havok and Banshee grew up and moved on with their girlfriends.
I don't think it is difficult to explain at all.