Originally Posted by
Trubo
In the scenario you described, yes that person will get saved to your Heroic ID. It will not work if the person you're trying to share the Heroic ID with is already locked to a different Heroic ID of that instance so if they wanted to kill anything before Thok it would have to be Normal only.
Note: While it isn't much a concern in current content you should be careful about this sort of thing when SoO becomes CRZ-able and still has the guaranteed Kor'kron Juggernaut from Heroic Garrosh as people will take Heroic lockouts and proceed to use them to get the mount, screwing over anyone else locked to that same Heroic ID.