Quest progress and dialogue are not, for instance. That version of the mod has quite a few caveats on what is and isn't synced between players. Skyrim Together previously released nightly builds of its in development version, which you can still learn how to access in the Together team's tutorial up there.
Skyrim Together doesn't have a full release yet, but you can try out an older, less stable version if you're dedicated. As you can imagine, getting Bethesda's 2011 RPG to play nicely on a server is no easy feat, so Skyrim Together isn't exactly a stable experience just yet. It's envisioned as a fully co-op Skyrim for two to eight players.
If you want to co-op through your Nord adventures with friends, Skyrim Together is one to keep an eye on. Sure, The Elder Scrolls Online exists, but that isn't really multiplayer Skyrim.
