Some puppies just get so excited that's what they do. Start putting a harness and leash on her, or collar, ( I just prefer the harness) and get her used to people and socialized with other dogs as well. Take her out for walks,that way she'll also get used to more people,and get some exercise at the same time. She will most likely grow out of this.
That sounds like submissive urination. There is a lot of good info out there about it. Search it on google. Is your pup having trouble housebreaking? If it can't hold it at other times either, not just when excited, you may want your vet to look into medical reasons, but it sounds like submissive urination.
Is your puppy shy or timid, does she pee any other time, or just when people come to the door? If there is nothing physically wrong with her (you can get her checked out by the vet) then it could be submissive urination or it could simply be that she's excited and can't hold it. I had such a dog, and once in a blue moon, he'll stll do it if he gets overly excited, but it's not submissive behavior. What does she do when people come to the door? Does she roll on her back, and pee at the same time?