Well, I took a quick scan through the Book of Ruin. Chaos Helot and Genestealer Cult each have a sidebar that says their leaders & champions can do group activations, specifically 2 & 1. The Corpsegrinders don't, but I'm inclined to think that's an omission, not a deliberate rule change. Since we all know how careful
GW is when writing new... bwahahahaha
Oh, I couldn't type that with straight fingers. Of course it's an oversight. Just like Enforcers in the Book of Judgement don't have any rules saying they can do group activations either.
Ogryn Slave gang leaders do specifically say "Group Activation(1)", and champions don't have a group activation rule, so that is *probably* on purpose.
Hopefully when the next
FAQ comes out that will get corrected, but until then I'd say it's safe to play it that way.
ETA:
Old rulebook,
pg 57:
Group Activation
When a player activates their gang Leader or a Champion (or the equivalent rank in gangs that use other titles for these fighters)...
Core rulebook,
pg 101
Group Activation
Some fighters, most notably Leaders and Champions, have a special rule called ‘Group Activation (X)’ listed on their profile.