Can you just leave a que of people outside, will anything happen to them over time?

A queue is fine. From what I see they don't need resources yet and when an attack happens they just run like pregnant dwellers.

hmm good to know, seems better to leave em out there for a while then!

Between your radio and those that just wander up to the vault door you are limited to having 10 queued up outside (as reported by other Overseers). Returning explorers do not add to this number so you can have more than 10 in line waiting to get in.

I am not sure what happens if an eleventh new dweller tries to line up to get in though as I have never allowed the queue to get that large.

From gathering cards, I can tell you there will be a pop-up message saying there's no more room available. It'll only let you get explorers into the vault (or at least 1 so the queue can move on)