I don't know what to expect from my group job interview?

I have a job interview tormorrow for a company called Home Fundraising. Their basically a charity company who employ people to ask for donations. I'm nervous because I never work well under pressure and have never had a group interview before. Also they say the dress code is smart casual which confuses me as thats a mive oxy moron lol. Any help from anyone who has experienced this type of interview would be greatly appreciated.