I'm not sure about Leviathan, but I am in Siren and I use Party Finder (PF) and it works fine. Check it constantly and make sure you have no Chocobo out if you make an invite request as you're running to a mark. I can't tell you how many people I have asking to join a party who are riding their chocobos and then ticked off when they didn't get an invite. Clue to them: I CAN'T invite you, the system won't let me. I'm not explaining it if you can't figure it out on your own.