This works with any number of factions. Keep in mind you can be allied with two factions that are at war with each other though.

I hope they add this to the vanilla game
great mod

Unless the author comes and says otherwise (or you have have already tried), should work with more factions as long as the invite cooldown has expired. When people phrase things like that they usually mean you can do it more than the said amount (unless otherwise stated.)

only 2? i wish this worked with multiple factions, playing on a server with multiple people we all have our own factions it would be cool if we could all be friendly