I've never actually used the Share With Friends thing but when I log in with my gmail username & password, I don't have any error with username or password being incorrect.

If you're sure that you're getting username (what you use to sign into gmail, not your gmail address) & password correct, then post over at this other forum. You'll need to register there. Click on the blue link below. The LW developers are at that forum & they might be able to give you a few more clues
LimeWire Forums