IME, if a gate agent announces "Boarding will start in 5 minutes, we will be boarding as per group numbers ... etc.", then in 5 minutes nobody cares and we see a ZOO gate, regardless of whether the agent then "formally" invites Group 1 first, or everybody at the same time. However, if at the time of boarding the agent firmly and repeatedly mentions boarding as per group numbers and others to remain seated, then it usually helps and avoids the mess.
Then we have airports where the priority lane is clearly indicated, yet crowds of G3-4 fill it early enough, which only gets worse when the boarding process commences. Then of course, gate agents should request the crowd to clear the lane to give way to eligible pax as per group numbers, but at the same point, it's also a lot about human inability to read (just to put it a nice way).