Control group based on custom attribute

This could, depending on how many subscribers you have and how many employees in each, easily unbalance a control group.

That said, if you already had a subscriber identifier as a custom attribute on each record, you could define a control group based on that.

If necessary, you could hash that value to a single numerical value (like the random bucket number) and work with that. Even just a 0-99 number would let you easily carve out any reasonable control.

If you didn't already have a value, that hash could be done easily enough in liquid - perhaps triggered at point of user creation (or whenever the subscriber values are set).