Skyrim still gets close to 5,000 concurrent players at least once every day, and never falls below 2,000. That's a hell of an achievement for a decade+ old single player game in 2022, though it doesn't have to be now. Skyrim Together Reborn is a co-op mod that will let you play with up to 29 of your friends in a shared instance of the game, from prison cart to the highest peaks of the world; and everywhere in between.
This isn't the first time this mod has been released, but it's been completely reworked from the ground up into its new "Reborn" iteration. NPCs are now fully synchronized, as well as monsters and dragons, quests, inventories, and equiopment also sync well, and everything else should look seamless between every player. Better yet, dialogues are now synced -- based on the party leader -- so you'll be able to follow along with story elements together, too.
The biggest change could be that the mod no longer loads a save when you die, but respawns you nearby so you can get back into the action. If you die in a dungeon, you'll start back at the beginning, but the others won't be forced to reload with you.
The developers set the default difficulty of the mod to Expert, since Skyrim was never balanced for that many players, and it doesn't scale, so expect a challenge when alone, but it will be a cakewalk if you bring all your friends along. You can always fight each other, though, just remember to turn the PVP setting on.
You can download Skyrim Together Reborn from the official Mod Nexus page.