In its attempt to drum up new business, The Yankee Group, a Boston-based market research firm, has come up with a technique that it claims is producing sizable results for a small investment. The firm offers a free T-shirt to anybody who sends it an attendance list from a computer communications seminar or from a membership roster for a high-technology trade association. Yankee claims that it landed some 10,000 names of prospective clients last year -- and it only cost them 200 T-shirts.