This is info from RSPCA site.
Behaviour
In the wild, guinea pigs live in close family groups, and it is unkind to keep one guinea pig. Companionship is essential, but two adult guinea pigs that don't know each other may fight, so choose two young littermates of the same sex, a father and son, or mother and daughter. Guinea pigs have a basic need to graze and should have regular access to a grassy area. Guinea pigs also need to have their own sleeping area.