I may be wrong but believe that if you have a party for boss fates for instance you can also build up your meter and use limit break.
I swear i have seen it before, again just speculation as i normally always solo or go with a Fc member to fates.
Leves are shared with people if you are group.
The solo instances in this game are mainly the class quests which i think makes sense, some however can be joined by others.