You get invited to those events, you can't just show up.
As for what to wear: go casual but presentable. Don't wear a suit or anything formal, but don't wear sweatpants either. I'd wear dark jeans (blue OK, but preferably black), Converse-style shoes, and either a decent check shirt or jumper.
Remember, they're not looking for technical people who know everything about every device Apple has made over the last 5 years. They're looking for friendly, outgoing, relatable people that customers will like talking to. You can teach that kind of person knowledge about Apple's products, but you can't teach an übergeek social skills. Be calm, friendly and outgoing. Look like you're enjoying the video they play during these group sessions.
Working in the Apple stores is better than almost any other retail job, but it's still a retail job. Be prepared for that.