Is it possible to control the group parameter with an expression?

What I have is a bunch of spheres that have been grouped, then made into a fractured rigid body. Right now they all fall down together, but I would like to randomise when they fall, so I thought if I could create an expression in the rigid body solvers group parameter to say “pick a random group over time.”
then it would have the desired effect, but I can't get it going.

Anyone have suggestions for this?