We finally are able to play online, but now we have a new problem. Downloaded maps are not showing up in-game. The original game has 250 or so user-made maps, avalable HERE. You are supposed to just unzip the file and drop the unzipped files in the game's ADDON folder.
It worked fine back in the day, but now none of us can get the game to recognize any maps. I also tried adding some to the miss8 and miss16 folders (nitro-style), but this did not work either. We are using different versions of the game on different operating systems, and we are all still having the same issue somehow. Nitro Pack's addon maps work fine, but the original game somehow will not read the maps any more. This is mind-boggling... someone, help!

Okay, thanks to Lightfoot we have figured out this problem as well.
Once you upgrade to I'76 Gold, you can only use Nitro Pack addon maps. (If you have the GOG.com version, it is already upgraded to Gold, so you cannot revert, but this is okay).
So you download any Nitro Pack map from Lightfoot's Stolen Maps.
Then, you unzip the 8-bit files (discard the 16-bit files). You will have two files. One will end in .TER, this file is fine. The other file will end in .CBT, .RAC, or .CB2. Change the extension to .LVL. Now you will have a .TER file and a .LVL file. Copy this file into your game's MISS8, MISS16, and ADDON folders.
Now it will show up and you will be able to host or join any game using this map.
The game does not work online by itself, so you have to use Tunngle: Tunngle Download Link. Once installed, search "interstate" in the search box and join the Interstate 76 room. From there you can play with anyone in the room. Just host by going to Melee -> Multi Melee -> Host -> Internet. Then add a server with your Tunngle IP address (found in the lower left-hand corner of your Tunngle window), pick your map and car and host the game. Anyone who joins will need to be in the Tunngle channel. Joiners will go to Melee -> Multi Melee -> Join -> Internet, and add a server with THE HOST's Tunngle IP. They should see your game right away and be able to join, assuming they have the same map installed.