The Hivemind is the new secret mount in Tides of Vengeance. Initially, the mount looked like the Brain of Yogg-Saron (model), however it was later changed to look more like a jellyfish with tentacles.
The Hivemind mount acts as a vehicle however only the last people you completed it with are able to join it. The mount's movement speed will be increased by 10% for every player that joins the vehicle, which is also shown through the Joined Hivemind buff.
If you want to link with different people, you just have to complete the final parts of the puzzle again with a new group.
All Puzzles for the Hivemind have been solved! You can check the guide here: https://warcraft-secrets.com/guides/the-hivemind
If you want the TLDR of this secret then check MMO-Champion's guide here: https://www.mmo-champion.com/content...d-Mount-Solved
If you want to join for future secrets then you should join the WoW Secret Finding Discord, the community dedicated to solving the puzzles, riddles and various mysteries the game has to offer.