The beisutsukai are members of a fictional society, also known as the Bayesian Conspiracy, depicted in a series of short stories by Eliezer Yudkowsky. Characters in the story include Brennan (usually the viewpoint character) and his teacher Jeffreyssai.
Derived from "beisu", a Japanese transliteration of Bayes (though more accurate Japanese pronunciation would be bei-zu, not bei-su), plus "tsukai", Japanese "user", therefore "Bayes-user". A casual, shortened pronunciation would probably come out "bayes-tskai".