If they've sent you an invitation and are asking for a CV then it's the first stage in their recruitment process. If you want the job, you should of course go along! What exactly they do is hard to say - but it will likely include some information about the job (and after all, even if you call this "advertising the job to me", you probably want to know what you'll be doing before you do it!), and presumably some sort of group interview or selection process.
Whether or not you can find the time with revision to do is really your decision - I would probably say that if you can't find a few spare hours then you could probably do with a break anyway!
As far as the other people there are concerned, I'm sure there will be at least a fair number, and there's no reason to think "everyone else is a prospective store manager" - after all, they need far more sales staff than managers!