Media were barred from the invitation-only event, held at a club in Indian Hill, the upscale suburb that Armstrong called home.
Armstrong sons Rick and Mark were to give eulogies to their father. Also expected to speak: businessman and friend Charles Mechem; and Sen. Rob Portman, R-Ohio. Astronaut William Anders and NASA Administrator Charles Bolden are on the guest list. A public memorial will be Sept. 12 in Washington with a final resting place still undetermined. President Barack Obama ordered flags to fly at half-staff to honor Armstrong.
Earlier in the morning at Cincinnati Children's Hospital Medical center, two fellow lunar pioneers spoke about their work in helping launch a children's health fund in memory of Armstrong, whom they praised as an inspirational team player and humble hero.
"America has truly lost a legend," said astronaut Eugene Cernan, calling Armstrong a hero who "came from the culture of our country," growing up on a western Ohio farm, flying combat missions, and then joining the space program.
Cernan and astronaut James Lovell recounted visitingU.S troops in Iraq and Afghanistan with Armstrong, saying he always was an inspiration when meeting troops, schoolchildren and other admirers around the world.
Armstrong was a great American who never capitalized on his celebrity and just "wanted to be a team player," Lovell said. Armstrong said any of the astronauts could have been the first to walk on the moon, but Lovell and Cernan said Armstrong was the right choice for the way he handled becoming an icon.
"There's nobody that I know of that could have accepted the challenge and responsibility that came with being that with more dignity than Neil Armstrong," Cernan said.
Relatives described Armstrong, who largely shunned publicity after his moon mission, as "a reluctant American hero."
Cernan was the last astronaut to walk on the moon. Lovell was commander of Apollo 13, where an oxygen tank in the spaceship exploded and the moon mission was aborted.
Lovell and Cernan said they had visited Armstrong two months ago at his home. He cooked breakfast for them -- and burned the eggs, Cernan said.
"Neil Armstrong was probably one of the most human guys I've ever known in my life," Cernan said.
Armstrong's family has suggested memorial contributions to two scholarship funds in his name or to the Neil Armstrong New Frontiers Initiative at Cincinnati Children's Hospital. His wife, Carol, is on the hospital's board.
Shane DiGiovanna, 14, an aspiring aerospace engineer with a rare skin-tissue disease, joined the astronauts Friday in their session at the hospital. DiGiovanna is able to hear because of a cochlear implant, a device developed by a NASA scientist.
Before the announcement, Shane, who said Armstrong always has inspired him, quizzed the two astronauts about details of their missions. Lovell recounted the streams of oxygen that wrapped their spacecraft "like a cocoon" after the tank explosion. The harrowing Apollo 13 flight was recounted in his book and depicted in the popular movie in which Tom Hanks played Lovell.
Cernan told him he was disappointed that the U.S. manned spaceflight program was halted, but predicted that Americans someday would return to the moon, and that Shane's generation would reach Mars.
Armstrong was raised in Wapakoneta, Ohio, and developed an early love for aviation. He served as a U.S. Navy pilot flying combat missions in Korea, then became a test pilot after finishing college. Accepted into NASA's second astronaut class in 1962, he commanded the Gemini 8 mission in 1966.
He then commanded Apollo 11's historic moon landing on July 20, 1969. As a worldwide audience watched on TV, Armstrong took the step on the lunar surface he called "one giant leap for mankind."
After his space career, Armstrong returned to Ohio, teaching aerospace engineering at the University of Cincinnati and generally avoiding public view for most of the rest of his life. He married Carol Knight in 1999. He had two sons from a previous marriage.
Friday night, two student groups at the University of Cincinnati will gather with telescopes to view the moon and hear some of the astronaut's former students speak. The moon will be full, the second time in August that this will occur.
This relatively rare event, called a blue moon, won't happen again until July 2015. The moon remains its usual color, but the astronomical event is associated with the phrase "once in a blue moon."
In announcing his death, Armstrong's family requested that when people "see the moon smiling down at you, think of Neil Armstrong and give him a wink."