You can already attack anything on the over world without being in party. Just follow them, let them hit the monsters first, and then you can one shot them. They will get full EXP... at least that's how it used to be last time I PL'd a friend from level 1 to 20.

So that takes care of the EXP issue. Beyond that, all you can really do is level another job with them, unless you've got all 80s.

But honestly, if your friends are having that much trouble in the overworld, then they've either got a gearing issue or they're not engaging with the game fully.