Let's say you are runner-up for an award and get to attend the ceremony where the actual winners will receive their award. You decide to go, but you don't want to go by yourself, so you invite a friend who might find this to be an exciting experience because he will meet "important" people (career-wise). In addition, your friend had also applied for the award, but didn't make it into the last round. You figure this way he gets to join some of the fun.
When you get the formal invitation, it gives suggestions (not instructions) about attending the ceremony. It says that you can take a guest (which you obviously knew) and that most people take the professor they are working with most closely. What would you do?