As discussed in another thread, FFXIV doesn't lend itself to the large community style events that we run in the main Gaiscioch chapters. It's just a different style of game.
That said, there are big benefits to grouping up! Here are a few I'd suggest we try:
Grouping for XP Chains (2-8 people)
You get the same XP for killing a mob solo, or in a group. And the same with XP chain bonuses. The difference is that groups can chain kills faster, and for much longer. HUGE xp bonus if you are killing lots of mobs.
Grouping for Gathering XP Chains (2-8 people)
Same idea as above, but while gathering. I've never tested this so I don't know if it works... but if it does, WOW!
Edit: I was able to verify that this doesn't work. Oh well!
Jumping the Queue - Duty Finder
If you are a tank or healer preparing to queue for Duty Finder, invite some DPS from the guild before you queue. You'll still get your fast queue time, and your guildies will get to jump the long DPS queue.