Alter Aeon FAQ (Frequently Asked Questions)

FAQ 7.1 - What are the advantages to grouping?

Grouping allows characters to acomplish things they would be unable
to do by themsleves, such as killing of a large mob for exp or
equipment.  Grouping also allows characters to specialize (hitting,
tanking, casting, healing, etc.) in the group making better use of
the skills availible.

You can also gain more experience in groups, especially if you are
the leader.