All share the queen's birth date — April 21, 1926.
The queen has laid on a three-course meal for 70 women and 29 men who, like her, will celebrate their 80th birthday on Friday.
"It has always been a standing joke in the family that I've never been invited to the queen's birthday party, what with us being born on the same day, never thinking it would actually come true," said Betty Kay, of Doncaster in northern England, one of the invited guests.
Public celebrations of the queen's birthday will be held in June.
Why does the Queen have two birthdays? For practical reasons.
Traditionally, British monarchs born in winter months would have their parties spoiled by the poor weather and so it was decided that each would have an actual birthday and an official birthday — either the first, second or third Saturday in June.
Prospective banquet guests were nominated by themselves or relatives, and 99 were chosen to represent all parts of the country.
Edna Richards, of Wolverhampton, said she was a firm supporter of the monarchy and admired the queen for maintaining a busy schedule.
"I think she is a marvel. I don't know how she does it all. Being the same age I know how she must feel," Richards said.
In advance of Elizabeth's birthday, Britain's Royal Mail on Tuesday issued a set of eight stamps featuring informal photographs taken throughout her life. All show her smiling.
There are no pictures in the set from the 1920s, when the queen was a baby and a toddler (and not yet in line for the thrown), and none from the 1990s, a decade in which three of her children's marriages ended and a fire ripped through Windsor Castle, in 1992 — the year the queen dubbed her "annus horribilis" (horrible year).
Amid the excitement surrounding the birthday celebrations, the queen's cousin told the British Broadcasting Corp. that she is "perfectly sure" the monarch will never abdicate the throne.
"It's not like a normal job, it's a job for life," Margaret Rhodes was quoted by the BBC as saying.
Buckingham Palace said Rhodes' remarks reflected the longtime position of the queen and there was no change in her decision to forgo retirement.
"These are the questions that are asked around these big events," said a Buckingham Palace spokeswoman speaking on condition of anonymity in line with palace policy. "There is no question of the queen abdicating. She will carry on as normal."