If you know that they have come to visit someone specific, but you just don't know who, then you can say "Who is it that you are here to meet?" or "Who are you here to meet?"
If they have come to visit a variety of people and even the visitor does not know who it would be good to talk to, you might make some suggestions "I can introduce you to the CEO, the accounts manager, the secretarial staff - who would you like to meet first? Or is there someone else you would prefer to meet?"
- For Teachers