This last Sunday's Sanctum (gamebreaker.tv Rift video cast) was talking about Raid Rifts and wondering about how they would work. Nice to see how they will avoid the grifing. Now what I expect is multi guild alliances, or a single guild with several 10-man groups, going out to open several rifts and in each one group will open it while the other groups support from the outside, making the encounters ezmode.
As I understood it, the more people you have participating in a raid rift, the more difficult it becomes. Examples of this might be that if you have more people healing, mobs might hit harder. If you have more people dpsing, mob armor/resistances might increase, or maybe their max HP will scale up to account for the extra damage.
As far as griefing, I can already see the Trion forums erupting with PvP players in an outrage over being denied the opportunity to interfere with opposing faction raid rifts. Personally, I kind of see it from both sides. I did my time on a PvP realm in WoW and was forced to the conclusion that my idea of "fun" open world PvP and the opinion of others didn't mesh. My focus was PvE with PvP on the side as an added element. That approach was best served on a PvE server with instanced PvP when i felt like it. But at the same time, if you take this segment of endgame that would "traditionally" be instanced and put it into the open world, a lot of people who really do enjoy their open world PvP but also place considerable weight on undisturbed endgame raiding would be kind of stuck making a choice they probably don't want to make.