What type of position is the group interview for? How many people show up? What type of questions are being asked? What kind of answers are you giving? Where do you sit (front or back)? How do you dress in comparison to others attending? How much research on the company or Hiring Manager do you conduct prior to attending?
Bottom line, group interviews give companies a chance to view candidates in a different dimension than one-on-one. How will you react when questions are asked? How do you acknowledge the other candidates while they are speaking?
If it were me, I'd dress appropriately (call the Receptionist if you aren't sure), come with extra copies of my resume, bring a notebook and pen, and I'd sit in front, ask good questions, and address the Hiring Manager by name (Mr./Mrs Smith, not "Ted").
Also, don't hesitate to politely exit the interview if it appears like the situation or the company isn't a good fit for you. It helps you establish control, which reinforces your value. It also alerts the company that you might be someone they can't have and that can frequently make you appear as though you are more valuable than the rest of the cattle that stayed behind in the room. Note, if you do get up and leave, have a meaningful answer and be professionally polite as you do so.