Here is how you and your friends join your Skyrim Together server. Each player plays on their own copy of Skyrim and keeps their own character and saves; the server keeps everyone in the same world.

What Every Player Needs

Skyrim Special Edition on Steam or GOG, updated to the latest version. The Anniversary Upgrade isn't needed. Skyrim VR, the Legendary Edition, and the Xbox Game Pass and Epic Games versions don't work with Skyrim Together.
Skyrim Together Reborn in the same version as your server, with Address Library for SKSE, installed with Mod Organizer 2 or Vortex. See Install Skyrim Together.

With Auto Update on (the default), your server installs the latest Skyrim Together release when it starts; with it off, the server keeps its version (see Update Your Server). To check its version, open the Console in Bropanel: the server shows it every time it starts, for example Server v1.8.3 started on port. Players on another version can't join. Get the matching version from Nexus Mods or the developers' GitHub releases.

Connecting With the IP Address Recommended

1
Install Skyrim Together if you haven't already.
2
Log in to Bropanel and select your Skyrim Together server.
3
Copy the server IP address and port from the Overview page, for example 123.45.67.89:27015.
4
Start Skyrim with your Skyrim Together launch option in Mod Organizer 2 or Vortex, and load a save. With a new character, escape Helgen first: don't connect during the intro.
5
Press F2 or Right Ctrl to open the Skyrim Together menu, then click Connect.
6
Paste your IP address and port into Address. If your server has a password, enter it in Password.
7
Click Connect. The chat tells you once you're connected.

Always enter the port. Without one, Skyrim Together tries port 10578, which isn't your server's port.

Connecting From the Server List

With Public Server on, your server is also listed under Public servers in the Connect window, by its server name. A server with a password isn't listed, so private groups always connect with the address.

Playing Together

Quests sync through parties. One player opens the Skyrim Together menu, goes to Player Manager and clicks Launch party, then invites the others from the player list. That player is the party leader: only they should talk to quest NPCs and loot quest items. With Auto Join Party on, players who join later go into that party by themselves, unless your server is a Public Server: there the leader invites them.

Troubleshooting

This server expects version ... but you are on version ...: install the Skyrim Together version your server's Console shows, or update both: players to the latest mod, and your server with Auto Update on.
The password you entered is incorrect.: check the Server Password, or the Admin Password if you joined with it, in Bropanel.
This server is full.: raise Max Players.
This server has ModPolicy enabled.: the player's plugins don't match your loadorder.txt. The message lists the plugins to install or remove. See Mod Check.
This server disallows ...: see Allow SKSE and Allow Mod Organizer 2.
Could not connect to the specified server.: check the address and port, and that your server shows as online in Bropanel with Server started in the Console.

How To Connect To Your Skyrim Together Server