The other day I was feeling particularly helpful (and on a recruiting spree), so I decided to check the Royal Jelly Palace and the Ironclaw Munitions Factory for any parties, as I also was trying to get an Antigua. Here's what happened:
-I first went to the Jelly Palace, clicked Join Party on Normal. Nothing.
-Switched it to Advanced. Got into a party of previously 3.
-Did my run, got a recruit, etc.
-Went to the Roarmulus Twins.
-Joined party on Normal and Advanced: Nothing. When I hit Elite, I got into a party of previously 2.
My point is, it's rather annoying and a pain in the neck to have to change the difficult two times just so you can join someone and help them. Unless there's a way to do it and I've been a complete moron for the past few weeks. So, if there isn't, perhaps you could readd that function?
I'm pretty sure there is an option to join a party, regardless of difficulty unless they took it out (i know for a fact that this option was in when difficulties first came into the game). Take a look at the mission screen again.