When you play, your character and any equipped gear gain experience points as enemies
are killed, whether you do the actual killing or not. You can play in groups to increase
the number of enemies that are killed and to level up your equipment more rapidly.
Note that the specific equipment level appears to be determined by the type of weapon
a teammate used. For instance, if your teammate uses a primary weapon, then your primary
weapon is the one that will gain experience. It’s also worth noting that you don’t gain
those points if you’re not within a fairly limited range, so it pays to stick close to
your friends to maximize your own benefits (and to enable teamwork, of course).
