You had time to look that stuff up online. I hope you did some research.
Dressing appropriately shouldn't even need to be mentioned. Don't fight, don't be combative. Hopefully you've researched the company too, because knowledge is power and you never know when knowing something will put you ahead of the group.
I had a group interview once, for a programming internship at a mutual fund company. Most of the other candidates were dressed business formal (suit, and usually includes heels for women), and a couple were on the more formal end of business casual (like nice shirt and suit pants but no jacket). If you've got a suit and heels, probably best to wear them.
It wasn't exactly laid back, since people get nervous when they interview, but I didn't notice any hostility between the candidates. Which kinda makes sense - acting competitive looks like "not a team player" to the interviewers. We talked to each other when we weren't in interviews or presentations, but mostly about school and other jobs we'd had.
We each got interviewed individually by two different people who worked there as computing specialists of some kind (so we were getting interviewed by people in our own field rather than HR people - the HR interview had been over the phone before we were invited), and there were a couple presentations about the company. I'm told that on-site interviews sometimes include a tour, but we didn't do that. (It's possible financial places don't because of security issues.) During lunch, we talked to some full-time employees who were just out of college, and the higher-up people left the room for that part - basically giving us a chance to ask questions without anyone who could affect the hiring decision present. We also got a few chances to just make conversation with the people who interviewed us.
I don't know if this goes for all companies, but for that interview, they seemed to partly be giving us a chance to evaluate them, and be sure we really wanted to work there. That was the only thing that was really unexpected to me. (Well, that, and that the "group problem solving activity" turned out to be about marshmallows and spaghetti and so I'd been lugging around my laptop for nothing... had been expecting they'd make us try to program something as a group.)