This is the exact same thing as with Chaos Dreads - you only roll for fire frenzy/berserk if they're on the table, as long as they're in reserves they're fine.
Also, since you roll for reserves at the start of the movement phase, you're not on the table for that turn's start of the movement phase and, again, don't have to check.
So the first time Grots coming out of a WWP would have to worry about Berserk Frenzy would be the turn *after* they came out of the portal - make sense?