They will join Judge Sir Martin Moore-Bick for the second phase of his investigation, which is due to start hearing evidence later this month.
In a written statement to MPs confirming the news, the Prime Minister said: “To ensure that the inquiry panel itself also has the necessary breadth of skills and diversity of expertise relevant to the broad range of issues to be considered in phase two, and to best serve the increasing scale and complexity of the inquiry, I have decided to appoint an additional two panel members to support Sir Martin’s chairmanship for phase two of the inquiry’s work onwards.”
It comes after members of Grenfell United started a petition calling for people with “relevant background, expertise, experience and a real understanding of the issues facing those affected” to sit alongside the judge.
The petition was led by Adel Chaoui, Karim Mussilhy and Sandra Ruiz, who all lost relatives in the fire.
It was signed by more than 156,000 people .
Mr Chaoui said: “This is a huge step for all of us who are desperate to make sure the inquiry delivers truth and justice for the loved ones we lost in the fire and those that survived.”
He added: “This panel means that the inquiry will have the expertise it needs to get to the truth of why our community was not listened to when we raised concerns, why such a dangerous refurbishment could be allowed to go ahead and all the issues that led up to that awful night.
“These are questions that need to be answered to make sure this never happens again.”
Meanwhile, Downing Street confirmed that a painting of the blackened tower given to the Prime Minister will hang in No 10.
Mrs May met a number of Grenfell survivors on Thursday evening, among them artist Damel Carayol.
Mr Carayol gave Mrs May the painting, entitled Grenfell Tower: Eyesore!! Final Straw, telling Sky News that the artwork was a “painful expression of anger”.
A Downing Street spokesman said the painting would go through the usual process for any artwork given to No 10 to decide “where best it fits”.
Asked if the art would be displayed somewhere in the building, he said: “Yes, that’s my understanding.”
On Monday MPs will hold a debate on the fire, which destroyed the west London tower block in June, killing 71 people.