The peeing is called Submissive Urination. It's actually a behavior from wolves that shows itself in pretty much every breed of dog out there, depending on the situation. Wolves (and dogs) have a very complex body language that tells you how they're feeling at any point in time. Basically, with this urination, your dog is saying he's very happy to see you, but that he knows he's the lowest animal in the pack. It's a very natural behavior, but not one people are very fond of!
In order to curb the behavior, unfortunately, you need to go against every instinct you have as a primate! First, stop acknowledging the dog any time he is in any sort of submissive posture. This includes crouching, rolling over, slinking, etc. Wait until your dog is in a more calm, self-assured manner to give any sort of praise.
Along with that, do NOT under any circumstances scold or correct your dog for submissive urination!! All it does is confuse the dog (the peeing is completely involuntary) making him even more upset that YOU'RE upset, which makes him want to submit even more! Once you stop reacting to the urination, you should see an improvement almost immediately.
Eventually you just get used to recognizing the submissive postures that indicate urination's on its way. This goes a long way to stopping the behavior entirely. We had the same problems with our dog, and he'll only let a couple drips out every few months or so now!
You want to make sure you're very consistent with all this, too. Let people know before they enter your home about the issues, and what to do about them.
In addition to working with the times the dog is actually peeing, make sure you're boosting your dog's confidence and bond with you in other ways.
Work on obedience training and games that your dog can win, like tug with a rope. You want to give your dog lots of chances to succeed. This is a great situation where positive training will go a lot farther than corrections ever will. If you want further reading, check out books by Karen Pryor, the woman who developed clicker training and was the forerunner of positive training, starting with wolves, which filtered into dogs. It should give you a lot of great insight!