Common sense would mostly cover this -

- Be friendly and polite at all times
- Meet n greet visitor's how you yourself would like to be greeted (remember you are representing your company and first impressions last)

We have a remote door entry system and each time the buzzer sounds anyone nearby can answer it.  More than usual I answer it as it's near me and I am the only PA/admin in the office.  However my colleagues do help out if I'm on the phone or away from my desk.  We usually say hello, can I help you.  The visitor(s)will reply with their name, company and whomever they have arrived to visit.  Once they are buzzed in, I or whoever answers the door, will then meet/greet the visitor, ask them to sign a visitor's book for health and safety purposes and then escort them to whoever they have arrived to see.  

Receptionists deal with all kinds of people  and issues on a daily basis eg. deliveries, visitor's etc so it's good to know your staff and be informed who is in and out of the office at any given time.  If your receptionist is well informed, then it reflects when meeting and greeting clients eg., they can go that extra mile if a VIP is expected.

G