The pre-expansion patch has new built-in technology the developer team will be testing. The game will automatically create new instances in an intuitive way. If a player is near his friends, both of you will be in the same instance, without noticing what happened. There won't be any stutter or queues when this occurs. There will be no AFKing at a quest giver for turning in a quest as it was in Warlords.
DEMON INVASION PRE-EVENT DETAILS
The pre-Legion event has been tested out on Public Test Realms on June 9 (Source). Basically, every WoW expansion gets a pre-patch, where Blizzard is preparing for an expansion launch. Usually it features an event that is somehow related to events leading up to the expansion, culminating in a release.
* These zones will be invaded by minions of the Burning Legion: Tanaris, Westfall, Northern Barrens, Mulgore and Azshara.
* Each invasion is a scenario and any player ranged Level 100 - 101 can join it by moving in to an invasion area.
* Each scenario has 4 stages.
** Defense against a general invasion at a point, such as Orgrimmar's rear gate or Tarren Mill.
** Defeating two lieutenants and a commander at that point.
** Defeating various invading forces marked on the map.
** Defeating a boss NPC at the original invasion point that requires a Group.
* Completing the scenario awards a Feat of Strength called Stand Against the Legion.
** Should you complete the scenario in all zones, you will be awarded the Feat of Strength called Defender of Azeroth.
** During the scenarion, you will be rewarded with a total of 260 Nethershard.
*** The currency is used to purchase item level 700 neck, finger, trinket, back items and set transmogs.
Here's the scenario in action as being tested on PTRs.
Are you ready to defend Azeroth from the Burning Legions of Hell?