You're invited to dress "Hawaiian style" - the more colorful, the better. If you can manage it, locate fresh flower leis for every one in your group. This would be a very special touch.
Beg, borrow or rent enough low coffee tables so everyone can sit on colorful throw pillows on the ground.
Cover tables with heavy white butcher paper - or tapa gift wrap paper - and then decorate with a profusion of garden greenery. Centerpieces of pineapples or piles of colorful fruits will complete the picture.
Use paper plates instead of china and let plastic forks and spoons take the place of silverware. When its time to eat, everything goes on the table at once; there are no courses at a luau.