Players will be able to choose between warm-up, challenge, and nightmare difficulty. The lowest will sync everyone to a Prestige level around that of the previous Invasion and reward appropriate gear from that, while challenge offers no sync and places you at the level of the current content. Nightmare, meanwhile, will boost the difficulty beyond the current cap and offer greater rewards (with more information due in the future). So no matter how far you’ve gone, you should have a challenge option to fit your character for a group.
Source: Official Site