The organiser is still in charge while the event is taking place – you are responsible for ensuring that things happen as they should and are the on-site liaison for the venue management. If anything that you have arranged does not happen as planned take this up with the venue manager immediately – they will generally resolve things there and then, which is better than complaining after the event.
Make sure that your attendees behave responsibly – they are representing your Society/Club, the SU and the University – and that any problems that arise are dealt with properly.