Some puppies will piddle a bit when greeted by other dogs or people, even their own family. Although this can be frustrating, keeping greetings calm and low key is the best thing to do.
The AKC website has great tips for this very thing:
Health Check: FIRST, take your pup or dog in to the veterinarian to rule out any medical condition that may be causing the issue.
No Scolding: Since this is an involuntary biological reaction by your dog, you need to find out what triggers the behavior and modify the interaction accordingly. So remember to never scold or punish your puppy for wetting as this will make him more anxious and worried and will exacerbate the problem.
You can read the rest of the tips in the article at the AKC site.