The 19-year-old midfielder has already spent time in America during pre-season training.
Fire head coach Frank Yallop said in a statement: "Grant is a young, technically-gifted player.
"I watched him play in England, and throughout our pre-season training camp Grant consistently proved that he has what it takes to do well in the MLS.
"We look forward to him joining our squad once he has recovered from his injury."
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