According to Wikipedia, the countries who have joined so far are:
United States (chair), Albania, Argentina, Armenia, Azerbaijan, Belarus, Egypt, Hungary, Indonesia, Israel, Jordan, Kazakhstan, Kosovo, Kuwait, Morocco, Pakistan, Paraguay, Qatar, Saudi Arabia, Turkey, UAE, Uzbekistan & Vietnam.
Many other states and organisations have been invited and are considering their response including the Vatican city.
The only states to have declined so far are Sweden, Norway, France, Germany, Italy and Slovenia.
On 20 January 2026 Trump referenced the "Board of Peace" and said "the United Nations never helped me" as a reason for its existence, claiming his board "might" replace the UN.
The Guardian called it "a Trump-dominated pay-to-play club: a global version of his Mar-a-Lago court aimed at supplanting the UN itself," arguing that the body ultimately outlined bore little resemblance to what the United Nations Security Council believed it was endorsing. According to the newspaper, a charter circulated to national capitals two months after the resolution's adoption made no reference to Gaza, instead presenting the Board of Peace as a permanent global institution.
https://en.wikipedia.org/wiki/Board_of_Peace