You can farm solo, but it's far more effective to farm in a group of 5 since everyone gets their own loot for every mob. It's even more effective if you farm with a group made up of people from your same realm/realm group because then you can trade BoEs if you get one you don't need but someone else does need it.
BTW, this is addressed in my 99 guide along with almost everything which has ever been asked in the 90-99 forums about 99 twinking. I know it's a lot to read through, but it's pretty thorough.