Complex sentence: Mountain was first sponsored by Variflex in 1981 before joining Powell Peralta a year later, where he formed a strong friendship with Stacy Peralta, the team leader and director of the Bones Brigade video series.
Split into simpler sentences: Mountain was first sponsored by Variflex in 1981 before joining Powell Peralta a year later. It was during his time with the latter that he formed a strong friendship with Stacy Peralta, the team leader and director of the Bones Brigade video series.