NEW YORK (Reuters) - Private investment firm Thomas H. Lee Partners is expected to announce on Monday a deal to buy a majority stake in Refco Group, the world's largest independent retail futures brokerage, for about $2.5 billion, sources close to the situation said. T.H. Lee's investment would give the New York-based brokerage more liquidity and possibly set it up to enhance its value through a public offering at some point in the future, analysts have said. Refco, which has operations in 14 countries and $20 billion in assets, has expanded and diversified through acquisitions like its purchase of...