make sure to separate your new piggy in a separate room to avoid any contamination and wait at least 2-3 weeks to make sure your new guinea pig has no signs of illnesses or you can also get them checked out by a vet most visits can run from $25-50 and if needed medicine the cost for that would prob be around $40-100. Once their cleared you can put a cage divider or set up a cage right next to the others so that they can interact and see each other and if all goes well you can introduce them in a neutral territory that none of the piggies have been in. If all goes well then their ready to move in together but make sure that all sent is removed from the main cage. GOOD LUCK!!