In a town where who's for dinner is far more important than what, among the most sought-after guests along Embassy Row are the 16 members of the Senate Foreign Relations Committee.

Night after night, somewhere in that land of damask-voered tables at least one of their number is sitting down to become the focal point of a game that's been going on since nations started doing business wth each other-diplomatic lobbying.

Last night, your average Thursday night in Washington, for instance, nearly all of them were on the scene at one of two locations.

At the Moroccan Embassy, it was Democrat Richard Stone of Florida, guest of honor with his wife, Marlene, at a party given by Ambassador and Mrs. Ali Bengelloun. The ambiance was relaxed, almost intimate, as the 38 guests, seated kneecap-to-kneecap at low round tables, dipped into the Moroccan national dish of couscous and roast lamb.

Across town at the Pan American Union, the spotlight was on the committee's new chairman, Idoho's Frank Church (D) and his wife, Bethine. Theirs was a dinner given by Secretary-General Alejandro Orfila and his wife, Helga, on behalf of the Organization of American States-a rather grand event for 180 black-tie guests, among them most of the committee.

If there was a contrast between the Bengelloun and Orfila styles of entertaining, their purposes were similar.

"Somebody would ask why the ambassador of Morocco would develop his relationship with a senator from Florida" Bengelloun said in his toast to Stone. And then replied, in the language of diplomats, to his own question: "I think it is natureal. Morocco and Florida have many similarities-the same sky, palm trees and beach."

At the Pan American Union, where 26 Latin American ambassadors to the OAS hoped to get beeter acquainted with Church and his colleagues, Orfila put it another way:

"The Senate Foreign Relations Committee is extremely powerful."

Powerful enough, certainly, to bring out an impressive representation of Washington officialdom and glitterati. And to bring down from New York Orfila's longtime friedns from their days together in Moscow, the Walter Cronkites.

Church, rushing in only a few minutes late from the Senate, called the week "the acid test," one of the most demanding yet in the two months he has chaired the committee. Back from a trip to China with four others on the committee, Church talked of being "immensely struck by the magnitude of China-the size of its Great Wall, the Forbidden City-and the magnitude of its problems."

The needed a little time to sort it all out, he said, and as for Taiwan, an issue his critics have accused him of flip-flopping on, he said he and Vice Premier Ten Xiaoping "simply agreed to disagree."

Also on that trip was another guest, and member of Church's committee, Sen. Edward Zorinsky (D-Neb.), who told of running into a lot of congressional people he hadn't expected to see. "I began to realize we could have had a joint session of Congress" in China, he commented. When Egypt's Ambassador Ashraf Ghorbal came through the line, Church enevloped him with a hearty bear hug. "We'll stand behind you," Church told him, an apparent reference to thhe forthcoming "peace package" which has just been reported out of committee and which Church says is the next big foreign relations push.

At the Moroccan party for Stone, the newly signed peace agreement between Israel and Egypt was also under discussion. "Morocco has been the only Arab country which has consistently supported Sadat and Israel," said one guest, "and that's why Stone is here."

But the issue more on the Moroccans' minds was their own, involving the use of American military equipment in the troubled Spanish Shara.

Asked if any progress had been made, Bengelloun said, "Ask the State Department - we're talking about that," but would not comment further.

As guests, including Associate Justice Potter Stewart, American University President Joseph S. Sisco, Tunisisan Ambassador Ali Hedda and columnist William Safire listened, Stone told dinner guests that Morocco was "a very significant place led by a very significant person."

But his real punch line came when he startled everyone, including his host, by telling them that the first U.S. senator from Florida had been a Moroccan who had emigrated in the early 19th century.

"The year was 1845," said Stone, "and the name was David Yulee Levi." CAPTION: Picture, Sen. Frank Church, left, with Helga and Alejandro Orfila; by Gerald Martineau