I'm a bit confused with how saving to an instance works in WoW. For instance, say on Tuesday I clear 5 out of 8 bosses in Dragon Soul. The next day, my group isn't around and I'd like to finish the last 3 with another group. The other group hasn't cleaned any of the content up for the week. Will it screw them up if I join them? I understand I won't be able to loot the first 5 boss drops that I already dropped that week, but can I still join them to get the last 3 if they're going for all 8?
How exactly does "saving to an instance" work?