Need to report to the DOT
This is one of several reports suggesting United end runs around denied boarding compensation rules. Important you report this to the DOT. They need evidence of a patter to take appropriate action.
Everyone in your party is entitled to denied boarding compensations. It appears this should be a check for 4 times the lowest fare paid for this flight among economy class passengers.
Note that even the passengers later allowed on the flight are entitled to compensation. Once they were issued a ticket on the later flight, they were involuntarily bumped. By DOT rules, the compensation needed to be issued on the spot. The fact that they later managed to get standby accommodations is not material.