How should I address a PI at the holiday party tonight? Our lab is having its holiday party tonight at a somewhat fancy hotel restaurant.  How do I address a PI (Principal Investigator) of our lab?  I’ve never met her this semester – she spends time at another research institute and communicates with us via group emails.  

Given that a party will be a casual meeting, how should I address her?  By her first name, say, "hi Karen", or is it better to address her more formally, such as "hi, Dr. Samsonite"?  Our lab consists of about twenty members.

Update

So I met her just a short while ago in her hotel room where she is staying through the weekend, and I decided to adress her as I normally would, thanks Aeismail for the useful tip.  She seemed just fine with it, so a big thanks to all for the answers and comments.  Party time!
Address her as "Dr." unless she invites you to call her by her first name.