The only achievements I can think of off the top of my head that require more than 1 person are Only the Penitent in Firelands and the Blood Queen one in ICC. Only issue I see with them is the fact you have to find people that want to do it, sometimes easier said than done. I could have sworn there was one of the achievements for a glory of the meta achievements that Blizz removed, maybe I’m remembering wrong.
Only one I really take issue with is the Only the Penitent one because it requires a minimum number of people for it to count. Think it was 6 unless Blizz changed it recently.