"Let me know if it crops up again" is fine if you're talking about a problem or something occurring.
- I have fixed the problem with your computer but let me know if it crops up again.
Here, it means "if the problem happens again".
We don't usually use it to talk about people in the way you used it for "people cropping up in the company". I would say "turn up", "appear" or "join".
The most common usage would be when referring to a subject being discussed occasionally.
Her name crops up once in a while.
The topic of the £100 Christmas bonus cropped up at just the right time, because he was in despair about his financial situation.