If they want to level for real, I honestly wouldn't bother. Between higher base XP, FoV, RoE, XP rings or bonuses, etc, they can basically follow the zones outside whatever city they start in (I pretty much always did Saruta->Tahrongi->Buburimu).
If you want to powerlevel them, then just have them to get to 10 and either head to a nice GoV or to Seekers areas. Offhand, 10 is high enough that they should still get XP with a level 99 in the party. For GoV, all they need is to get XP at all, and it counts to finish the objective. Another good one for this is the Cnest donut. The high level mobs down there don't aggro (elementals will, if there's weather, but they're low level), and there's a high density of them with a couple objectives that work, depending on which side you focus on.
You can also just go to Seekers areas, or keep that mind for later. Base XP is high enough there that, even after being reduced, it can still be decent for someone leeching.
Either of those will likely remain a better way to XP, for relatively short sessions, than having you cleave in Abyssea or something (a full XP alliance is different, I mean for leeching off of a single high level).
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