Those guys standing upstairs were checking transport tickets. If you're in Prague and if you're going through Mustek metro station I recommend to have a valid ticket. They are checking it here almost all the time.
Speaking of public transport. If you need help with anything regarding Prague public transport (tickets, how to get somewhere, etc.) you can call free phone number (00420) 800 19 18 17 and they will answer your questions. I saw their ad in a tram yesterday and since it was written in English you don't have to speak Czech to get helped.