Opening nights were not invented as an excuse for benefits and parties, it just seems that way. Tuesday night, the Dance Theatre of Harlem opened at the Kennedy Center, and the Doll League, a black women's community activist group, used the occasion to raise funds for the Juvenile Amputee Clinic, associated with D.C. General Hospital and the Dance Theatre itself.

Diplomats, representatives of the arts community and leaders of the black community each paid $500 or $1,000 for the performance and an after-theater buffet in the Rotunda of the British Embassy. The 100 or so guests, including Reps. Charles Rangel (D-N.Y.), and Louis Stokes (D-Ohio), enjoyed a buffet of baked ham and shrimp re'moulade and waited to congratulate the company, which arrived later.

Arthur Mitchell, the company's founder and director, and prima ballerina Virginia Johnson arrived first. Johnson, a graduate of the Washington School of Ballet, was spun around the room by old friends. Ann and Phyllis Reed, who grew up with Johnson "right around the corner--neighborhood," wanted to know if she was going to stay with the company. "Oh yes, I want to keep dancing forever," she said, smiling a smile that didn't stop all evening.

Mitchell seemed happy and virtually speechless. He had laryngitis. "The company looks better and better all the time on the Opera House stage," he managed to say. "It's becoming a home for us."

As the evening proceeded, a bit of protocol was observed--and it became an impromptu political poll. Patrick Hayes, managing director of the Washington Performing Arts Society, rose to toast the Dance Theatre for its "beauty, love and inspiration." As the guests were, he noted, "on British soil," Hayes proposed the traditional "Long Live the Queen." "Hear, Hears" and a lusty repeat of the phrase followed. He then proposed a toast to President Reagan. When the response was somewhat less full-throated, good-natured laughter swept the room.

Rangel, whose district includes Harlem, summed up the evening, saying, "Arthur, for years I have been trying to explain to presidents what Harlem is all about. Your group does that better than I ever could."