Robert J. Myers, former chief actuary of the Social Security system, was in Grenada studying its new social security program when leftist Prime Minister Maurice Bishop was slain during a military coup eight days ago.

Because of the ensuing curfew and the impossibility of a flight out, Myers said, he spent the next few days at the "magnificent beach" behind his hotel and visiting American students at St. George's University School of Medicine next door.

The Reagan administration has said that fear for the lives of students and other American civilians was a major reason that U.S. forces invaded the island Tuesday.

"We saw a lot of the students," Myers said. "Some were frightened and wanted to go home . . . . Other students told me, 'We've put a lot of money into this, and I want to stay and get my degree.' I heard that there was a poll of the students and most wanted to stay.

"The students told me the chancellor of the college met with the new government . . . and was assured that it wanted the school to continue operating," he said.

During his confinement to the hotel and surrounding area, Myers said, there were no guards to enforce the curfew, and nobody was threatened.

Two things that struck him most sharply, he said, were the favorable attitude toward the United States of people and officials he encountered before Bishop's death, and the Cuban Embassy's strong denunciation of the slaying.

Hired by the Organization of American States to provide technical advice to Grenada on running its National Insurance Scheme, Myers went to Grenada Oct. 13 with his wife.

Bishop was killed Oct. 19, the day Myers had been scheduled to leave. All flights out were canceled, and a four-day curfew was imposed.

The curfew was relaxed for the day last Monday. Myers and his wife got a car to the airport, took a charter plane to St. Lucia and came home. The invasion started the next day.