Horrible Problem: Cat marking territory on carpet!

Does anyone else have this happen? We have four cats, but we know which one it is (after my husband set up a web cam to catch her). Previously I thought it was a territorial problem between the cats, but we've started noticing a pattern: she does it after we have company. Needless to say, once she starts this up, we don't have company for a long time! And worst of all, she's urinating, not spraying, so it's in our new carpet. It's a constant clean-up battle, although I think she's stopping now that she's made her point after last weekend's visitor. She's very much a "scaredy-cat." We got her at the humane society as an adult, and we think she was mistreated in her previous life. It took her years to warm up to us, but she's still very scared of other people. Besides never having company over again, any suggestions on how to get her to stop?? Kicking her outside forever isn't an option here.