The unknown here is whether the crew members were on BP3 tickets. If they were "deadheading" that would mean that they had been scheduled by UA to get moved from Station A to Station B by UA on BP3 tickets and were entitled to board before the OP. In such cases they need not be in uniform unless they would have to work immediately after arriving at Station B. If they were "commuting" then they would be travelling BP-8 and should not have been boarded before the OP.

In the future the OP should always verify his BP status at the gate. Then if a boarding priority deviation is perceived to be occurring, the OP need simply ask "What was the BP of those folks you just boarded?".

Things used to be much easier before all employee ticketing went electronic. When all employee tickets were paper, the different types of tickets and their BPs were easily discernible. Now we must rely on the GA to enter BPs correctly and then to process the DM list appropriately.

Of course, screwing with the DM list is easily auditable and has been the road to dismissal for more than a few GAs.