The bulbapedia page is a little confusing. It says it's based on the user's base attack. So if you were using for example Weavile, it has 120 base attack so would the power be 120/10 + 5 = 17? Or does it use Weavile's actual attack stat at that point, e.g. 300/10 + 5 = 35 from each Pokemon?
And what about the Pokemon on your team attacking? Does it use their base attack stats, or only the one who started Beat Up?