A player asks this question to Blizzard "Will characters on accounts without BC updates be able to interact with characters with BC updates in the non BC zones?" and Nethaera responds with this answer:

Yes, you will be able to see them, trade with them, group with them in non-BC areas (if you don't have the expansion) and you will even be able to use items from the expansion provided it's brought into the main part of Azeroth and you are the right class or level to use them.