If you're not familiar with Southwest boarding, it's based on a first-come, first-serve check-in basis. Instead of reserving a seat, you simply lineup at the time affording and then pick a seat as you enter the plane. The position where you line up is based on your check in time and unfortunately, Southwest has not implemented a system where Companion Pass passengers can be guaranteed to board together. They state that they are working on such an implementation but it's been a while since they've been working on it, so I'm not sure when it's going to become a reality. You can find out more about the Southwest boarding policies here.