A close relationship with all of your bros.
A very comfortable group of bros that frequently spend time together. Usually consists of 3-5 bros. One person will usually have 1-2 bropacks, any more is somewhat uncommon. Common activities of bropacks include sleepovers, watching movies, driving around, eating, playing Halo or other multiplayer console games, and pulling pranks.
Alternatively, one who has no bropack but still hangs out with different arrangements of groups is a "One-man bropack". For whatever reason, this person was not compatible with other forming bropacks and/or could not merge with existing bropacks. This could be due to personality, parental restrictions, or a multitude of other bropackability issues.
-One-man bropack: "Hey, I'm free tonight, wanna chill?"
-Bropack member: "Can't bro, I already have plans with my bropack."
A group of bros traveling in a pack just to be seen by all and look cool. Typically sporting bro-tanks, wayfarers, and similar chach-gear. It was originally designed as a weapon to infiltrate the infamous "pack of girls". However, since the metrosexual movement, it has advanced to become a method of garnering attention by displaying bro-machismo.
Look at that bro pack walking by...what a bunch of douchebags.