If you search this forum you will find information on introductions (Rat Behavior section). Female to female is usually easy. As far as I remember you should not introduce a very young female to an older one. I think it's advised that the new rat should be at least 12 weeks old.
It's definitely a good idea, rats are generally much happier when they have other rattie companions. Just make sure you read up on intros like Malarz said and it also might be a good idea to get two new young females instead of one since there's always a small chance intros won't go well and you could end up with two lonley rats.