Helloo!! I have been working at my local Pets at Home for about a year now, so I have been through this process too. You have nothing to be nervous about, the group interview is intended to be very informal, casual and fun. I will run you through what happened at my group interview.
Upon arrival there were four of us, and we firstly introduced ourselves and told eachother about any pets we had (I have a dog). Afterwards, we were introduced to some colleagues, and they ran us through some of the job description, hours, pay, etc. After this, we got a group task - our task was to decorate an empty childrens' fish tank (with gravel, ornaments, plants, whatever from the shop floor. You do not need to know anything to do with fish although it may help, I had never previously owned fish). This part is really to show your skills working in a team, and apparently I was chosen at this point as I had 'leadership skills', basically no one had spoken so I said something like "right so who wants to do what? do you two want to choose the ornaments and we will create a name for the tank?" It's about confidence to speak and be involved.
After this we were individually interviewed (a very short interview just chatting about why you'd like this job, why you'd be good, just chat). We were told that there were 2 jobs available, and that the two of us would later receive a phone call to then come back to the shop for a second longer (half an hour) interview and job trial. Apparently though once you have reached this point the job is basically yours. The trial really isn't a trial, it is just being shown how to work the till and shown around the shop. The interview was more formal but also still very chatty and laid-back. For the trial I wore a plain white t-shirt and black trousers. So that really is the entire process!
It is a great job in all honesty, the usual shop job responsibilities but with an added twist, you really do learn a lot and it is not as boring as many usual shop jobs. Best of luck and I am open to any more questions if you wish