When you get there and are waiting to go in/waiting for the interview to start, take a scan around the room of your competition. This will distract you from being nervous about the interview and you get a rough picture of what you're up against.
Have an interesting prepared fact and/or hobby to tell the group. You will be hard pressed to go to a group interview where you don't have to stand up and say your name, interests/hobbies and a fact about yourself. Tailor your hobby/interest to the field of the job you are going for. For example, you're interviewing for a job at a book shop and you like to write. Interviewing for a job at a music store? You might like to collect records.
Sometimes in group interviews they'll pair you up with someone, you talk to them for five minutes finding out basic facts (like the ones in number 2) and then you introduce them to the group. LISTEN TO THE PERSON YOU ARE TALKING TO. This is a test of how well you listen and engage. LISTEN.
Mental Mind Map
Once everyone has introduced themselves, form a mental mind map of people. Usually only half of the people there will be a threat to you getting through to the next round. Doing this can help calm your nerves if you're worried about being up against so many people.
If it's a retail position, have a prepared story about customer service. Doesn't matter if you are the customer or the server in this story, just make sure you communicate that you understand what good "customer service" is.
Don't be too chatty or too quite. Be a fence sitter. Speak when spoken too and raise your hand when questions are asked, this makes you look assertive. Don't just yell stuff out or speak over others. That is rude.
If a friend or someone you know is there, don't spend the whole time talking to them. It's highly obvious when you're speaking to someone you're comfortable with and spending the whole interview with talking to your pal makes you look bad. Get to know some of the other people in the room.
Sometimes you'll be broken up into smaller groups to do activities. When this happens, try not to be too bossy just be assertive. There's a fine line between showing leadership/assertiveness and showing that you're a control freak.
If they make you act out a customer scenario there is no need to be worried about it. No one is going to remember it. You can put your money on the fact that the other interviewees are not even listening to you.
Smile. If you have resting bitch face just smile every now and then. Other wise it looks like you don't want to be there and have no interest.
DON'T ASK STUPID QUESTIONS. Any information you can gather from their website, the job description or a simple Google of the company's name is a stupid question. Also don't ask about time off, THIS IS NOT THE TIME.
A good question might be, "what do you enjoy about working here?"
Bring your resume (and/or portfolio). Even if you've sent it to them, just bring it, makes you look prepared.
Wear appropriate shoes. Nothing says, "I'm an incompetent human" like not being able to walk in your shoes.
Eat before you go in. There's nothing more awkward then having a rumbling stomach in an interview room.
Don't wear tights as pants. (This isn't specific to group interviews, but is also a general life tip)
Say thank you and goodbye. Don't just run out of the room as soon as it's done (common sense).