How to stop my puppy from weeing when she gets excited?

My 5 month old black Labrador wees every time someone new approaches her and sometimes even when her other owner (my girlfriend) comes home from work! She is wagging her tail and greeting them but urinating at the same time. It's not the end of the world, but when we take her (the puppy) to other peoples houses and she wees on other peoples carpets it's a little embarrassing. Is this just a puppy thing that she will grow out of or is it a problem we have to adress with training or the vets? Many thanks!