Beat Up deals damage from each Pokemon on your team that does not have a status ailment (or is fainted). The power of Beat Up is calculated as:
(User's Base Attack ÷ 10) + 5.
Beat Up will also receive any boosts from the originating user, such as Same Type Attack Bonus (STAB) and any applicable items. Although each Pokemon on your team supposedly attacks, the actual power and effects of each hit are based on the original user only - any items held by your team do not affect the damage dealt.
This is it. The actual power depends on the attack power of the pokemon using it, then you multiply by the number of pokemon in your party. So let' use this situation as an example:
Dugtrio used Beat up!
Dugtrio has a base attack of 80, so using the formula (User's Base Attack ÷ 10) + 5.
(80 ÷ 10) = 8+5 = 13
Beat up will have a base power of 13 for every attack, and since you have 6 pokemon, 6 X 13= or 78 base power in essence. This is how the damage is calculated in Generation V.
Prior to Black/White, Beat Up dealt 10 base power typeless damage from each able Pokemon.