The Bostonian Group has leased 21,281 square feet at 500 Boylston St. in Boston’s Back Bay, expanding from its 19,318-square-foot office, also in Back Bay.

The Bostonian Group, a Marsh McLennan Insurance Agency, is relocating from Four Copley Place in Boston. After the expected move in May 2011, the Bostonian Group will occupy a portion of the third floor.

Barry Dube and Amy Unangst will oversee the project management and relocation planning for CresaPartners, according to a statement.

Rick Lowe, Brandon Leitner, Joe Doyle, and Joe Sciolla of CresaPartners in Boston represented The Bostonian Group.

The sublandlord, Houghton Mifflin Harcourt Publishing Co., was represented by Gil Dailey of Cushman & Wakefield and Matt George of Equity Office.

According to Lowe and Leitner, The Bostonian Group wanted to upgrade its space and location, accommodate its future growth and take advantage of favorable market conditions.