Its works the same way Pre-6.0 Heroic worked

Its works the same way Pre-6.0 Heroic worked. You can't join other heroic(now mythic) raids and the ID is bound to everyone and if you quit at 4 bosses and they continue to 10 and you rejoin, you'll be locked to 10. It's nothing new and its surprising to see so many people confused.

Unless these are all people wow gold guide who never did pre 6.0 heroics and didn;t know about the lockouts.

- No Emmi. Heres an example You do the first 5 bosses with your mythic group You get ID - 1234
You leave but want to try to join someone else at the first 5. You can't because their ID is 4321. Its all Raid ID based. you cant join a fresh mythic or in progress. Only your ID.