--Pigs that already have been introduced and get along well.
Also, a side note: Google really isn't the most reliable source for guinea pig info, so I'd recommend just searching right in this forum instead. There are TONS of threads out there that might hold your answer! Just a thought for when you have future questions.
I'm not sure how it works with piggies (I hope to get mine Saturday if the hurricane stays away lol) . I have 3 ferrets. They are all bonded to each other but there are times (as with any cage mates) that two get along better at one time. My male rough houses and with the smallest girl. When she gets dragged around the oldest female will go bully the boy away from her. But just as easily they will all snuggle together in a sleep sack. I'm guessing it works the same in piggie world. One may be "getting on the nerves of the others" and be banned from the little group until they fall in line.