In an emergency, and a fire is certainly an emergency, people inside a building are required to exit using the designated doors, stairs, etc. Once outside the building, everyone has to move to an area named as a fire assembly point which can be found by the various fire exit signs around the office.
Fire safety officers are clearly identified by wearing a coloured safety vest and it is their responsibility to conduct a roll call, a check as to who has gathered and remained at the fire assembly point. Remaining at the fire assembly point until directed to leave is essential and just because a siren stops sounding, that is not a signal to leave the fire assembly point.
But while staff will be aware of the evacuation procedure, visitors to the building may not. This is why visitors are required to sign a book and list their details. This book can then be used to check if everyone has vacated the building. Every business has to have a fire safety officer and drills or rehearsals for a real emergency are held from time to time.
Locations of fire assembly points are clearly marked at the sites themselves and on wall diagrams within the building and in literature provided to staff. If new buildings are added to the site or if fire assembly points are changed, full details must be published and a drill carried out.