Attending Doctoral Dissertation Oral Defense?

It really depends on the school. At my university, defenses were public, and mine had very good attendance among students (even some undergrads). But some schools make them private. It's a good idea to ask a grad student; most of them should know if you can go or not. No, participation isn't expected; you can ask questions if you're confused, but usually it's a good idea not to - unless it's a very simple question, defenses are extremely high-stress situations, and you don't want to trip up the defending student with something they might not remember or didn't spend much time researching and were hoping no one would ask about. There's no special attire in the US (in Europe, it can be very formal, academic robes and all). Usually what happens is that the student will give an overview talk of their research and professors will ask questions, often related to the subject but not necessarily. It can last a few hours, so don't go if you have to leave early.