http://bulbapedia.bulbagarden.net/wiki/Beat_Up_(move) and http://pokemondb.net/move/beat-up report that in BW, Beat Up no longer uses the base Attack stats of your teammates, but uses your base Attack stat (along with items like Choice Band) to calculate damage. After testing it on the Smogon University server, I know that our server uses the DPP mechanic where it uses the base Attack of each of your teammates. So either Bulbapedia and PokemonDB (the only other website I could find with detailed information) are wrong about a new mechanic, or it should to be updated on our server
Edit: I'm going to test it ingame tonight
Edit 2: Confirmed, from my tests I've seen that Beat Up no longer takes into account the base Attack stats of your teammates, which means the current PO mechanic is indeed invalid
Edit: I'm going to test it ingame tonight
Edit 2: Confirmed, from my tests I've seen that Beat Up no longer takes into account the base Attack stats of your teammates, which means the current PO mechanic is indeed invalid