Merrily exchanging gibes in the Terrace Theater last night, Beverly Sills and Donald Gramm turned the National Opera Institute's fourth Annual Awards Evening into a friendly family affair, which indeed it was. Servants and lovers of opera abounded.

Receiving awards for service to American opera were baritone Sherrill Milnes for his work with Affiliate Artists, Inc.; director Kirk Browning for his many TV opera productions; Metropolitan Opera archivist Mary Ellis Peltz for her long and devoted service; and the New York City Opera for its presentations of American opera.

A special category of trustee awards this year honored Vivian O'Gara Weyerhaeuser, founder of the Metropolitan National Auditions; and, post-humously, Robert Joy Collinge, former Baltimore Opera manager and an active force in Opera America; and, also posthumously, former Metropolitan tour manager Francis Robinson, and recording executive Dario Soria, ardent promoters of opera through records and television.

Entertainment kfor the evening was provided by the 10 young artists who will receive this year's career grants from the institute. Slected from more than 200 entrants, the winners, who sang excerpts from opera and American musical theater, were sopranos Margarita Castro-Alberty, Evangelina Colon, Charlotte Ellsaesser, Linda Helm, Karen Rambo, Joy Simpson and Debra Vanderlinde; tenor Carroll Freeman, countertenor Melvin Earl Brown and baritone J. Patrick Raftery -- given a particularly enthusiastic reception as a "local boy" from Silver Spring.