Set up some bluetooth and rfid sensors near the bar and
(a) periodically poll to see which IDs are nearby
(b) query the neighborhood when known IDs purchase liquor
(c) send messages to people in the neighborhood
bluetooth mac addresses can be associated with users at other places where they might use rfid and be able to confirm their phone, otherwise track their phone
have some display terminals somewhere else that display who is near each bar, both for being able to find that person and to allow a person to use their rfid tag as an identifier and have a bluetooth message sent to a person to ask that they get them a drink also (additionally track that this person asked this bluetooth id for a drink)