An undedicated server is one that you must be in for others to join. This tutorial describes how to host your own undedicated server for others to play on.

Step 1
The first thing you need to do is make a separate Tribes directory for the server. Don't use one that you play with. It shouldn't have any special scripts or configs in it. If you would like a stripped down copy of Tribes, you can download one here: Tribes
It is possible to use one with configs in it, but things like custom models can cause users to be unable to play on it.

Step 2 (Only if hosting a mod)
Now we need to tell Tribes which mod to host. Right click on Tribes.exe and click Create Shortcut. Now right click on the new shortcut and click Properties. In the Target Path box, add -mod <name of mod folder> at the end of the path.
For example, if I wanted to host an Annihilation server, my target path would be: C:\Modding\Tribes.exe -mod annihilation
Click OK and you're done.

Step 3
The final step is to run the new shortcut. If you are using Windows XP, you will need to click Unblock if a Windows firewall dialog box comes up asking if it should continue blocking the application. When the shortcut has been executed, instead of going to the Join Multiplayer page, click Host Multiplayer. On the next page, you can choose the starting map, a server name, a password, and if you want to record a demo.

Step 4
You may need to open some ports to allow Tribes to be seen by users outside of your network. Follow this tutorial: Port Forwarding