Does anyone know if there are any mods or methods that allow multiple people to play a community single-player map with multiple people. I haven't been able to find anything, but I figured if there were any, someone here would know of them. Thanks.

You may be able to contact the creator for a vmf, or hunting for the bsp, decompiling it, and getting the vmf. From there, you could add the multiplayer entities to the map and play it like a multiplayer.