Oh wow. How many tournaments have you gone to and not known this, Sart? Whip out your rulebook for a moment, flip to page 78-79, the rules for Stubborn start at the bottom of 78. Toward the end: I don't think it needs explaining beyond that, it is pretty clear.
Also on pg. 79 it says that if the mount or rider are ITP, Hatred, Frenzy or Stubborn, the whole model is. Ergo, Ancient Steg is Stubborn means Preist/Chief is Stubborn means any unit they join get to use their stubborn Ld. So an EotG in a Saurus unit would have Ld8 without modifiers and no lower than Ld6 (the stubborn Priest) with modifiers.