There are two types of job interviews commonly called group interviews. For one type, you'll gather with other job candidates to meet with one or more interviewers. The second type is more accurately called a panel group interview or just a panel interview. It's where two or more interviewers team up to ask you questions. Descriptions of both types follow.

Group Interview
The simplest form of group interview is little more than a presentation about the company conducting it, perhaps with open discussions and question and answer sessions. It's also the company's chance to initially screen candidates, by observing how individuals behave and stand out among their peers. Interviewers will likely also observe

Attire, manners and body language
Communication skills
Group interaction and participation

But it's a two-way street: It's also your chance to observe and screen, to decide if you want to work for the company.

Other matters aside, group interviewers may favor candidates who have meaningful questions, because it shows that the candidates are truly interested and maybe worth one-on-one interviews later. It's also one of the good ways to stand out among the group, as some candidates will arrive unprepared. To help you formulate questions to ask, study the job description and research the company beforehand. Natch, it's also a good idea to prepare otherwise, as you should for any interview.

That'll help you to stand out too.

Some companies take a group interview to another level, by conducting exercises that simulate a work environment. For example, the interviewers might split the group into teams, and give each a work-related, hypothetical situation or "case" to resolve. The interviewers will then ask the teams to present their results (e.g., in role-plays) in front of the whole group or later as individuals.

During group interview exercises, interviewers might closely observe and eavesdrop, ask and elicit questions, take notes, and maybe even interject a twist or two. Although the interviewers formed teams, you can bet that they're scrutinizing individuals. The skills measured during these exercises include

Interpersonal
Persuasion
Communication
Teamwork
Leadership
Organizational
Stress management

Group interviewers might be looking to see who takes charge, how well he or she delegates tasks, and how the other members react to his or her leadership. They might also observe how well all individuals

Improvise
Handle stress
Plan
Involve other team members
Give constructive criticism
Take criticism, constructive or otherwise
Assist in resolving the issue at hand

And so on. During group interview exercises, it's better to be among the leaders than the followers, or at minimum, an active participant vs. a casual observer.

As indicated, a group interview might consist of exercises designed to test one's improvisational skills, in addition to others. Studying behavioral- and case-interview flavored tips in advance might be helpful. But because group interview exercises typically consist of hypothetical situations, there are few canned questions and cases one can practice in advance, other than the common questions most interviewers ask. For the hypothetical, you must already have the answers on the tip of your tongue or the ability to manufacture solutions in your head. That's what it's all about.