I stand corrected on the Fearless special rule. As
Mr. Shine and
Yarium pointed out, if even one model in a unit has Fearless, the unit gets fearless. So even though the Grots don't have 'Ere We Go or Furious Charge, they are Fearless as long as the Runtherd is alive as
koooaei said.
There is a bit of sloppy editing on the wording of the warlord trait. It is worded two different ways (on p92 and p104 of Codex:Orks). However, in this case it doesn't matter because the Runtherd has 'Ere We Go, therefore is Fearless, and therefore the Grots are Fearless too as long as he's alive. Go Grots!