I may be mistaken, but I think you already have the basic grouping for lights, as it’s really one of the simplest rules.
Turn on
A list of lights
When a switch is turned on
Specify that switch
The only other thing we need that you might not have is a clause like the one in smart lighting:
Turn off as well. (this has to be optional because sometimes you won’t want them to turn off when the master turns off.)
That groups the lights. You can either pick one of the group as the master, or you can assign them to follow a virtual switch. The rule works the same.
But notice that this is super simple: the only thing it matches is on and off. Not set level or color or any other Device capability.
I think it’s likely that people will have existing groups that were created in other smart apps and are tied to a virtual switch.
But unless I misunderstood something, you’ve already given them the ability to create a simple group of the “follows master” type. 