Pakistan was admitted to the United Nations on September 30, 1947. Legally, independent India is seen as the successor state to British India, and Pakistan as the new creation. So India is seen as having participated in the founding of the United Nations.
After the Security Council unanimously recommended the admission of Yemen and Pakistan to the UN, the General Assembly took up the resolution; and Afghanistan was the only nay-sayer to Pakistan's admission. (The vote was 53-1-0). India welcomed Pakistan's admission; and I don't know why that vote doesn't put to rest the perpetual Pakistani paranoia that the Congress-led Government of India wanted to undo Partition.
The UN website carries details of the resolution and accompanying speeches.