If you want to server hop, you need a party member to have entered the zone before you AND that party member must be from a different CRZ/shard.
Previously: when you join a group, you are pulled to the leader's server.
Now: when you join a group, you are pulled to the server of the first group member in the zone.
This means that the first group member in a zone becomes the "owner" of that zone. Every other group member who later enters that zone will be pulled to the owner's server, regardless of character level. Ownership can be changed ONLY if a zone has NO group members in it. Then the zone goes back to being "unowned" until the next time a group member enters the zone.
1) Johnny - Area52 goes to Felwood for Minfernals. He sees three, catches one, and invites his friends Orkus - Hellscream and Dumass - Nagrand for the other two.
- Felwood is now owned by Area52.
2) Orkus - Hellscream is heroically AFKing through a World Boss and waits for boss to die before flying over. Dumass - Nagrand is nearby and flies to Felwood.
- Felwood is owned by Area52, so Dumass is pulled to Area52 server when he enters the zone.
3) Dumass sees the Minfernals and catches one. Johnny leaves the group to join LFR.
- Felwood is STILL owned by Area52, a zone changes ownership only if NO group members are in that zone!
4) Dumass flies to Darkshore to buy Withers while Orkus starts flying to Felwood.
- Felwood is now unowned, no group members are in that zone! Darkshore is now owned by Nagrand.
5) Orkus - Hellscream enters Felwood.
- Orkus is the first party member to enter unowned Felwood. Felwood is now owned by Hellscream.
6) Orkus sees no Minfernals, because there are none up on Hellscream server. Dumass flies back to confirm because one was still up as he left!
- Felwood is still owned by Hellscream. Darkshore becomes unowned as Dumass leaves it.
7) Dumass enters Felwood, but it's still owned by Minfernal-less Hellscream so he sees no Minfernals. Orkus yells at both Dumass and Johnny, then hearths.
- Felwood is STILL owned by Hellscream, a zone changes ownership only if NO group members are in that zone!
8) Johnny finishes LFR and is teleported back to Area52's Felwood, where the last Minfernal is still present. Johnny could now invite Orkus to a party and then wait for him to arrive to ensure that Orkus gets moved to Area52's Felwood, but he logs out instead.
Note the above is actually a simplified example, because CRZ and sharding exist. If Area52 and Hellscream were on the same CRZ then Orkus would have seen the last Minfernal because the zone owner is actually Area52 CRZ, which he would have been a part of.
While sharding is not possible to predict as far as I'm aware, you CAN determine which CRZ you're about to land in, which is based off datacenter for US license servers. See https://www.reddit.com/r/wow/comments/3 ... ur_crz_us/
For non-US license servers, CRZ should also be based off datacenter but I'll note that as I do not have an EU account, I have not confirmed this.