I know for a fact that Forsaken Guilds exist. I know the Royal Apothecary Society is probably one of the largest I’ve seen. I believe they’re generally on the smaller side and do a lot more RP in the world/Eastern Kingdoms, so I don’t personally encounter a lot in my regular RP.
In addition to Undercity united, you might want to check out the Shadow Sermon. It takes place semi-regularly in the Sepulcher.
That should be a good place to find a lot of Forsaken characters to start you search for a guild.