Here's some information copied from it:
Base recruitment chance = 0%. Modifiers are added to it:
* +10% / +5% / +0% / -5%, if the attitude is Friendly / Aggressive / Hostile / Wild
* +5% if the alignment (Light, Dark) of the hero and the neutral creatures matches
* +5% if the race of the hero and the neutral creatures matches
* +5% if the hero's army contains the same creatures (improved and unimproved are considered different)
* +5% if the neutral creatures are unimproved, and the hero's army contains their improved version
* +10% if the hero has the "Diplomacy" skill
* +20% if the hero specializes in these creatures.