How do you get guinea pigs to get along when they have never met before?

Hold the new guinea pig in your hands, getting the scent on you. Pet your old pig lovingly after letting him smell your new pig's scent on your hand. Bring your old pig into a neutral territory (his cage might bring out the territorial instinct,) and sit him on the floor or somewhere safe. Holding the new pig, watch CAREFULLY as you let your old pig come over to check out the situation. If he doesn't, you can try putting them about six inches away from each other, but be ready to remove one if it gets nasty. Keep them in separate cages for a few days, cages preferably close to each other so they can see each other, and gradually lengthen their together time. When they seem to get along (playing, popcorning, eating together, sleeping together; certainly not chattering at each other!) you can leave them unsupervised for a little while. Use good judgement. When you are sure they are friendly towards each other, you can let them be together permanently. Keep the extra cage in case of sickness.