from what i have heard (and experienced re airline group interviews) you go in, get a name tag and get seated at a table/in a group where there are a few of you and an interviewer. You hear a company prep talk, get to ask any questions and get asked a few general q's (ie name of ceo, destinations they fly too) anyone can answer but its good if you can answer one or two as it shows you have researched the company.
then you generally do a group activity... something involving teamwork where the interviewers can see how you react with others in a team or time limiting environment.
airlines each have their own variations of group interviews, some make you dance and sing some make you talk on a topic of their choice for 5 minutes, some you have to answer some essay questions. it all depends. good luck everyone!