The Together Team has announced that the “Skyrim Together Reborn” mod has a final release date of Friday, July 8, 2022 at 16:00 GMT / noon ET, and it will add multiplayer support to specifically Skyrim Special Edition on Steam, particularly version 1.6. Console and VR are not supported. This mod has been in development quite a while and was supposed to enter a beta first, but development has gone smoothly enough that the team is just headed for a full launch this week.
A wiki now exists with instructions on how to install the Skyrim Together Reborn mod for multiplayer Skyrim when it becomes available. The team cautions that this mod is “not perfect,” and for the best possible experience, you should probably play it with other mods disabled. However, you can enable other mods if you want and try your luck. The team is encouraging users to inform each other which mods do and don’t work. In any case, this multiplayer mod is primarily intended for two-to-eight players.
The Together Team had been working on a similar multiplayer mod for Fallout 4, but the team tightened its focus to just Skyrim Together Reborn in order to get it finished. The team essentially states in a FAQ that the Fallout 4 multiplayer mod is not likely to get finished at this point, unless additional developers volunteer their time.
For an idea of how the Skyrim Together Reborn mod will work, you can watch some of a video the Together Team uploaded this past May.