(1) He was her Lds 4th grade Sunday School teacher [perceived moral authority]
(2) He hired her for a job 4 or 5 years later, when she turned 14 (or 13)...and she was still employed at the time of the hot tubbing [real authority that had she been 18, may have qualified as sexual harassment...as it was, it went much further than that -- with her being just-turned 15]
So, Garn was presumably abusing his authority as an LDS elder or bishop, or whatever his position in the church, to seduce her.