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The 27-year-old, a free agent after being released by the Bees at the end of last season, has signed a three-year contract at the Community Stadium.
Us boss John Ward told the club's official website: "Marcus is a big, strong midfielder who brings power to that part of the pitch and can score goals too.
"He has a good pedigree and has proved he can cope with being at the top end of this division."
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