Hi claire
It can be a problem in young bitch pups, shes probably a little worried about meeting new people, having a wee is just a form of submission, saying i'm inoffensive. It may help to let your visitors in while Cassie is in another room, get them seated, then let her in. they are less of a threat when sat down, then get them to give her treats, making her think visitors are good to have around rather than a threat.
Jen