At the end of the day in terms of actual PROGRESSION through the game there isn't really a lot of opportunity for it unless you're all, like, totally in synch with where you are in the story. Grouping up would mostly be for running stuff after the fact. As has been said many times by both people and staff, FFXIV is seen as a Final Fantasy game first and an MMO second.
HOWEVER, once you reach the end of an expansion (2.0, 3.0, etc), all the content between expansions is all at level cap for that expansion. So there are a ton of dungeons and raids that are locked at 50, 60, 70, and 80. These are notable stretches of the game where you would be doing the same level stuff even if you aren't in the same place as eachother.
Ahhh, this helped my understanding a lot, thanks! (Especially the part about being FF first and an MMO second)
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Let the voice of love take you higher, With this gathering power, go beyond even time!