Where is the best place to meet up with others at the center? And, best drop off location?

I agree with the parking structure in front.

Depending on what you are seeing there it gets extremely busy out front near the main entrance and down the street since a line usually forms. I suggest meeting people at a bar or restaurant nearby on K St. After the show, its not so bad and you can meet people outside the main entrance near the light up art structure.

Parking structure in front