My employer called everybody into the conference room on Friday, in shifts, to inform us that all wages were being cut by 20%. Is this legal????

Probably. A contract or CBA could be an impediment. Trying to make changes retroactively is generally an impediment. Minimum wage for Non-Exempt employees would have to be honored, and the FLSA limit of $455/week for Exempt Salaried employees would have to be honored. Other that, the change is probably legal given the facts as you have stated.