The phrase "a first group" is correct and usable in written English.
You can use it to refer to a group of people or things that go first in a sequence or ranking. For example, "The athletes in the a first group competed in the first heat of the race."
Mr. Ajan said a first group of strangers visited the house eight days ago.
Ms. Trimarco said Venezuela had granted formal refugee status to a first group of 47 Colombians.
Shortly afterwards, a first group of more than 350 children were released by armed groups.
This was not just about his patrons at Godolphin, celebrating a first Group One success at this meeting since 2007.
He arrives here on the crest of a wave having come within centimetres of claiming a first Group race victory in France last month.
General Myers said a first group of prisoners would be flown soon to a new high-security jail at the American naval base at Guantánamo Bay, Cuba.
He provided the 40-year-old David Griffiths with a first Group winner and will try to follow up in York's Nunthorpe three weeks from now.
Earlier this month, the Guardian was among a first group of foreign journalists and Greenpeace activists who were invited to see how they live and operate.
Moses Josiah, a leading gospel-musical sawyer, was playing & pleased-looking staff people were leading a first group of children through the exhibits.
Police used teargas at noon when a first group of protesters, many of them workers who had come in from provincial towns, tried to break into the parliament.
The company has trained a first group of 150 militia members and is drilling a second group of equal size, an official familiar with the company's operations said.
