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 amount of enemies that are killed, and to more rapidly level up your equipment. Note that the specific equipment leveled 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).