The 18 year-old midfielder, who has been a regular in the reserves, has signed a six month deal with the option to extend after being offered terms.
Head of Youth Steve Thornber told the clubs official site: "Grant has got unbelieveable talent, is strong and has got pace. He's like a bull in midfield and If he has a good pre-season I'm sure Ronnie will offer him a longer deal.
"We need to see what he can do week in, week out."
MillersMAD wishes Grant all the best for the future.