Reinhart (6-foot-4, 207 pounds) played last season in the Western Hockey League, leading the Edmonton Oil Kings' defensemen with 12 goals in 58 games.
In 2010-11, Reinhart scored 25 points (six goals, 19 assists) in 45 games as a rookie with Edmonton.
The 18-year-old attended the Islanders' development camp in June.
"It's definitely a different experience, with the guys being a lot older and a lot bigger," he said then. "You go through the same kind of things in your first bantam camp; you have to meet a whole bunch of new guys. The concept is the same -- building relationships with different guys."
"He's been really big for me," Griffin Reinhart said. "Obviously he's gone through it all, and with the teaching aspect, he's been able to help with a lot of my game -- help me with my weaknesses but give me some praise as well."