As someone who works in retail and involved in recruitment - suit absolutely not essential, generally it doesn't hurt - but bare in mind that we would be looking for people who we'd consider would stick around so if your applying for a non-managerial position and come in done upto the nines in a suit and presenting an overly professional aspect then in that context wearing a suit would be over doing it and hurt your chances.
Appearance and presentation is important in a role where your public facing so basic grooming/hygiene, a good pair of shoes, smart trousers and a nice shirt will pretty much tick all the boxes anything further won't really help you, probably won't hinder you but see above. I reccomend black trousers and a tie but a smart pair of jeans and no tie won't really hurt your chances either aslong as you look presentable.
The biggest aspect in depending if you will get this kind of job or not will be basic competency and how well the people doing the interview perceived your synergy with the current team would be.
Group interviews will mostly be about seeing how well you present yourself - be polite, clear and confident but not too overly confident, you will probably be asked to introduce yourself to the group, give a little bit of background on yourself and probably be asked to do a little roleplaying or talking in smaller groups about relevant experiences, etc. so be prepared with some examples of how you've dealt with problem customers, feedback from colleagues, etc. you might have if you've worked previously.