i can just explain it here for anyone else who looks. you first download a VPN(virtual private network) profile to your iphone via the setup menu in the multiterraria app. then you close out of your apps, go to your settings, and turn your VPN on. then reopen the multiterraria app and you have a few choices from there. you can create a public or private game or join public and private games. if it says you have a connection error in the multiterraria app you just have to toggle your vpn off and on and try again. and like ps3 said yes when you switch to a private game on the join tab you must have a code before joining a game in the terraria app itself. when you create a game it gives you the code for your friends on the app. last but not least when you have chosen the type of game you want to play, you must leave your multiterraria app running in the background. and most importantly you must have patience. it takes a few spins before you can find anybodies world whether you have a code or not. i found it helpful to click join game then go back and hit join game until they pop up.
tip: if youre having trouble joining somebodies world via multiplayer try starting a tutorial world before you join your friends game. almost always works for me.