As tobeamiss said they are marking to claim their space. you do not say if your dogs are male or female or if any or all of them are nuetered/spayed which is also a reason for marking. Are they just outdoor dogs or have they been housebroken? If housebroken, they should be able to be trained to not mark on the patio just like they won't mark in the house. There are articles on the internet for dealing with marking behavior. One big issue is the sex of all the dogs and if they have been spayed/neutered.
Your guest should have to crate her animals so the gracious host doesn't have to suffer. It doesn't sound nice but if the visitor doesn't like it. They can also move. You have to try and be strong and insist. Also, vinegar is a great odor neutralizer but unfortunately won't stop the problem. They'll still go on that area to put their smell back and re-stake their claim. Good luck to you. :)
You could try spraying vinegar around the deck animals don't like that smell that might stop them from peeing.
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