Here is what I had done back in the old 2.8.4 Physcs:
mScene->setGroupCollisionFlag(PhysicsUtil::GROUP_AIVEHICLE, PhysicsUtil::GROUP_BARRIER, false);
The code above disable collision between all Phys actor in group GROUP_AIVEHICLE and GROUP_BARRIER. The function setGroupCollisionFlag is direct Physx function from class NxScene.
And you assign actor to group using function like below:-
actor->setGroup(group);
where actor is NxActor (Physx object handle)