Supervisor at work favorizes a "select" few....Found Letter I wasn't suppose to find!?

Hello, I have known for some time that my supervisor has a few favorites in our group, but recently, I found an email that she sent out titled "Shhhh....Select girls only" it was an invite to a party from my boss to only 4 girls, leaving the other 4 of us out. I feel this is wrong since she is in charge of delegating tasks and igning promotions/raises. I think I will take it to HR, but I know it will never remain confidential seeing as she blabs about everythinng. There was a guy who got in trouble for taking his shoes off in the office and she was telling all of us about his getting in trouble and punishment, ect. What would you do?