For the purpose of debugging (maybe even playing some day), this page explain how you can join as a client in a SS3D server, in or outside the Unity editor.
We don't have the SS3D Hub yet, so this is only valid before we have that.
First, launch a server by following instructions on the page Hosting a server.
Assuming you built the game and have a SS3D.exe file in the Builds folder, you can simply double left click on the Start SS3D Client - GhostOfBeep.bat to launch a client, with GhostOfBeep as a username.
This is only applicable for Built Executables.
Open a command prompt in your project's path to the Builds/Config, and run the following command changing the fields appropriately.
This will launch the client, replace clientUsername with your username and replace serverAddress with the desired server address, you can use localhost or 127.0.0.1 for a local server.
This is only applicable when using the Unity Engine to join a server, which should be avoided if you're not developing.
You can set the Network Type to Client, the Editor Server Address to the desiredserver IP, or your local IP (127.0.0.1) if hosting from the same machine, and the Ckey to your desired Ckey.
start SS3D.exe -ip=serverAddress -ckey=clientUserName -skipintro