Signature. Word of mouth. Asking players to help out with advertising. 

There are a lot of ways. Aside from the ones I listed, I.prefer to let the RP speak for itself. 

Then the players are free to talk about how good a previous iteration was and why people should seek it out. I imagine this is how Legend if Renalta is ran, by Brovo, though he can confirm or deny that. 

my approach apparently works as someone was pointed in my direction when (S)he was looking for a dark fantasy roleplay.