If the person isn't advanced enough for the class, the coach should recommend a spot where the student would be a better fit.
We got a new kid in our dojang a few months ago. He had moved here recently and transferred from another dojang, where he was a green belt. We put him in our green belt class to start. This kid was in way over his head. We're trying to teach the class things like tornado kicks and spin hook kicks, and he couldn't even do a back kick.
What we found out is that while our school goes White -> Yellow -> Purple -> Orange -> Green, his school goes White -> Yellow -> Green. So he's the equivalent of our purple belt (who would just be learning the back kick). He went to the Purple belt class and fit right in.
It does remind me of a funny story, too. We have a guy at our dojang who is in his 60s and speaks barely any English. Well, after our adult class on Saturday, we had our demonstration team. This team is only open by invite, in that every student there has been personally invited by me or my Master. Well, one day I see this guy come in for demo team practice. I just assume my Master invited him and start going through our demo forms. My Master calls me into his office and asks what he's doing there. Luckily we have a lady on our demo team who speaks his language, and she was able to ask him what happened. He just saw a lot of black belts in our demo team and assumed it was a black belt class.