Lemon Zest in Jaco is top notch. every plate is excellent- try the lobster franchise
Pelicano on heredurra beach, right outside Los Suenos is one of all my client's favorite- open air, very nice tradition styled Tico restaurant right on the beach
try the whole fried snapper with garlic butter
any Los Suenos restaurant can handle large groups, all have very good food, and service and are very convenient if you are staying in LS- Bamboo the asian/ sushi is my favorite
Hicaco, on Jaco beach is a very nice, upscale, Americanized version of El Pelicano in heredurra. with bigger menu. also open air, right in the beach. can easily handle large groups.
(but most people prefer Pelicano)