You have to go to a city to do it.
Either:
-
just being in the city allows you to join a group in a different layer, or
-
a specific NPC gives you a short buf that allows you to join a group in a different layer
The buf mechanism would work even better since you would then get a debuf preventing you from layer hopping again for the next N minutes (which of course grows considerably each time until it decays), so you can actually see how long you have to wait and can tell others if need be.
Why are layers so damn fluid anyhow? Since they act as your virtual realm, you should be stuck in the same one barring exceptional circumstances.
Only Blizzard’s internal algorithms should be able to dynamically move players to other layers as a balancing mechanism. No explicit action like just joining another group should control this, which of course players can and will exploit.