Is it panel group interview or candidate group one? If it's panel, that one is nerve wracking but like all other interviews dress nice, know your resume in and out, MAKE EYE CONTACT, and let your interest show when answering the questions. One question that you will typically be asked is tell me about yourself. I always hate that question. Another one I hate is tell me about your weaknesses (sometimes I wanna say delicious desserts? LOL). Keep in mind group interviews can run for hours or days.
For candidate one, the interviewers are assessing your teamwork qualities and problem-solving qualities. You don't want to be the one that's overly critical of the other applicants opinions or answers but you also don't want to agree with everyone too. Show that you can think individually but contribute your individual thought to the group.
Also if they ever ask you what do you do in a group? Best answer I got from one of my favorite professors ever is: "finish my work first, then help others when I'm done." It doesn't sound very team-work like but honestly tasks in groups are most often finished when everyone does their own part first and then contribute it to the group. Also it's a good answer because so far I've gotten into the 2 internships I did a group interview for.
Here is a website you can check out for possible interview questions! It's a very good site!: http://www.glassdoor.com/Interview/index.htm
Good luck!!!!!!!!!!! Be yourself and be honest. Honesty is good. My current boss asked me during my interview if I wanted the job to just get some money? I told her that yes that was one of the reasons. LOL. And she still hired me ....(not to say you should tell them you just want the money though! but if they ask then why lie? if they don't ask, you don't ever have to tell)
Nai_1_1 previously gave good advices.
Well, personally, I just would like to give my 3 little personal advices:
1/ Be well Dressed & Appropriate.
2/ Be Interested & Enthusiast by the job and/or company during the interview (Eye-contact is a must!)
Make a small research about the company you applied etc... Make & prepare your questions for them etc...
3/ Be Honest in your answers (including if they ask you about your strength & weaknesses)
If you don't get this job after your interview, don't give up ! See each interview & failure as an opportunity to learn & practice/improve your interview skills.