Grinwis, 25, joins after playing the past three years in USL. Last year, he featured for Saint Louis FC, and prior to that he played for the Rochester Rhinos for two seasons, where he helped them win the USL regular season title and USL Cup in 2015.
“Adam impressed in our pre-season friendly last season and has a track record of success in the USL,” Lions general manager Niki Budalic said in a team statement. “His athleticism, character and competitiveness will fit in well with our group of goalkeepers."
Playing collegiately at the University of Michigan from 2010-14, the Ada, Michigan native was a second team All-Big Ten pick and All-Big Ten Tournament selection in 2012.