After reading that Tempest Keep was doable solo, I decided to give it a shot with my two friends. First note: LOTS OF GOLD DROP. Great place to farm if you want to do something outside of Heroics and Cataclysm raids with just a few friends. I think you probably need between 4-5 people, at least for the last fight. Up until that point, the boss fights were fairly reasonable. We haven’t downed the last boss yet, but I hope to finish it up tomorrow night.
Another great reason to farm this is just for tier gear in preparation for the upcoming Transmogrification feature for patch 4.3. So far I’ve managed to get a few tier pieces. Of course, one of the best reasons to go is for the mount. With the low drop rate, you have several excellent reasons to asks others to come with you.
Beyond that, I think going for both personal and guild achievements are excellent reasons to bring people here. I don’t think you need that many if you have a bunch of 85’s, but just enough to DPS things down. At least, one tank and a healer (hopefully a shaman) to help you during some of the CC moments.
Finally, a great reason to do this is that it’s a good chance for your guild to practice as a team and get used to each other. My guild at the moment has mostly soloist who quests. They might do the occasional random or battleground. I’m certain though that people would love to try new things if presented with the opportunity.