My dog did this too when my first foster came, she was quite dominant and a wannabe alpha. With my subsequent fosters he has not done it, but they were more submissive. He is not very dominant, and also I think he is getting used to the new furry friends that visit and he is feeling more secure (perhaps?).
Just keeping a close watch on him and their interaction helped me to prevent marking, also I think I had to ensure that I was paying him a little extra attention every so often to make up for having to tether the fosters to me to housebreak them. Without that he was perhaps feeling ousted?