I've seen this called "submissive urination." There are probably two different things you can try -- one to address the physical and one to address the psychological.
On the physical side: send the pup outside to pee in anticipation of visitors as often as possible.
On the psychological side: build the pup's confidence with obedience training or other activities. From what I've read, if you have a truly submissive animal by nature, you might never be able to fully correct the problem.