Step 1: Clear up to the raid boss that drops the mount you want. Navigate to the group finder, click on premade groups, and then click on custom and start a group. Give it whatever name you want.
Step 2: Log out and switch to an alt, run the alt outside of the raid, and search for your own group in the custom tab of the group finder. Sign up for it.
Step 3: Log back onto your main, accept the invite request from your alt. Log back onto your alt, enter the raid. The lockout should now be transferred. Repeat with as many level 50 characters as you want. I’d recommend waiting for leadership to transfer to your alt, then kicking out your main and delisting the group. If you log back onto your main and it asks you if you want to be saved to the raid, teleport out and reset all instances. That should fix it.
As of right now, only level 50 characters are able to view and sign up for/create premade groups. As far as I know, the only raid that this doesn’t work for is Dragon Soul. Before and after the pre-patch, it’s worked for me with Mythic Blackhand and Ulduar. It’s also extremely useful for farming Tusks of Mannoroth, as you can clear Garrosh over and over with re-roll tokens on the same character.
I do have a video guide recorded to prove that it still works in the pre-patch, but I’m not sure if I’m allowed to link that as it’d be considered plugging my own content. So if you’d like the link to that video, feel free to send me a DM.
There’s also an Asmongold video guide from 2015 that showcases how to do it, but I’m not sure how out of date that is.
Feel free to comment any questions and I’ll explain as well as I can.