Make them required to deal at least 1 point of damage against the final boss. I rarely see people wipe in a strike anymore so it would mean that if they wanted any rewards they would at least have to physically move through the entire strike . At that point they may as well just play it anyway. Also this would mean that people who join in progress are not penalized in any way (If you want them so stay then you realize this is a good idea) and unlike some of the other suggestions where you need a certain number of kills in each zone if one person just demolishes the enemy his teammates aren't penalized for it.