BunnyFluff
You could add nodes inside of the building containing the information for the businesses (1 node per business in the general centre of the floor space they occupy), as for the flats/offices above I have not found a generally agreed upon way of mapping these, It's probably a safe bet that if there are also an address node with the addresses to the flats/offices above/the buildings address contains the addresses for the upstairs flats then its probably fine (see https://wiki.openstreetmap.org/wiki/Addresses#Buildings_with_multiple_house_numbers  for more information on how to map these addresses)