While not a lot is yet known about them, they are considered to be,

Sith Loyalists and/or the Sith Eternal!!

Sith loyalists were men who served and aided Darth Sidious and were strong supporters of the Sith Eternal. The assassin Ochi was one such person.

And according to Rise of Skywalker Visual Dictionary the Sith Eternal are

The Sith Eternal was an organization of cultists loyal to the Sith religion who continued the Sith belief and sought to resurrect its tradition. The Sith Eternal were followers of the Dark Lord of the Sith and Emperor, the Sith Lord Darth Sidious. The Sith Eternal spent years building the largest armada known to the galaxy, the Sith fleet, as part of the Emperor's plan of revenge to use the Final Order to build a new Empire. However, the Sith Eternal were ultimately defeated at the Battle of Exegol, preventing the rebirth of the Sith and ensuring their extinction.

_{Just making a note that there is not yet a strong distinction between groups (Sith Loyalists, Knights of Ren, Acolytes of the Beyond, Alchemists, Sith Eternal) that are all loyal to the Sith, as it appears many were serving the same cause, but it appears that The Sith Eternal are the source cause and are the ones likely situated on Exegol according to the latest updates.}