“THE Russell-Einstein Manifesto was issued on 9th July 1955 in London at a Press Conference held in Caxton Hall.
A few days after the publication of the Russell-Einstein Manifesto Mr. Cyrus Eaton sent a letter to Bertrand Russell offering to finance the Conference of scientists which was called for in the Manifesto, and suggesting that it be held in Pugwash, Nova Scotia. This offer was not taken up because at that time it was planned to hold the Conference in India, where Prime Minister Jawaharlal Nehru was very keen on the Conference of scientists and had offered hospitality for it.
It took about a year before the plans for the meeting in New Delhi were formulated. Russell asked the two other British signatories of the Manifesto, Cecil Powell and Joseph Rotblat, to help him in the preparation of the Conference. Eric Burhop, who collaborated with Powell in the World Federation of Scientific Workers, was also involved.
In discussions between these four, a preliminary agenda and a list of invitees, to make a total of about 20 participants, were prepared. It was agreed that the letter should be signed by Russell on behalf of the signatories of the Russell-Einstein Manifesto. The first step, therefore, was for Russell to write to the signatories to obtain their agreement to the draft of the letter of invitation; these letters were dispatched on 6th July 1956.”
You can read more about the origins of the Pugwash Movement here.