Answer by taking a quote from the following article:

Muhammad Ali (; born Cassius Marcellus Clay Jr.; January 17, 1942 - June 3, 2016) was an American professional boxer and activist. He is widely regarded as one of the most significant and celebrated sports figures of the 20th century. From early in his career, Ali was known as an inspiring, controversial, and polarizing figure both inside and outside the ring. Cassius Clay was born and raised in Louisville, Kentucky, and began training as an amateur boxer when he was 12 years old.

Ali said that he first heard of the Nation of Islam when he was fighting in the Golden Gloves tournament in Chicago in 1959, and attended his first Nation of Islam meeting in 1961. He continued to attend meetings, although keeping his involvement hidden from the public. In 1962, Clay met Malcolm X, who soon became his spiritual and political mentor. By the time of the first Liston fight, Nation of Islam members, including Malcolm X, were visible in his entourage. This led to a story in The Miami Herald just before the fight disclosing that Clay had joined the Nation of Islam, which nearly caused the bout to be canceled.  In fact, Clay was initially refused entry to the Nation of Islam (often called the Black Muslims at the time) due to his boxing career. However, after he won the championship from Liston in 1964, the Nation of Islam was more receptive and agreed to publicize his membership. Shortly afterwards on March 6, Elijah Muhammad gave a radio address that Clay would be renamed Muhammad (one who is worthy of praise) Ali (most high). Around that time Ali moved to the south side of Chicago and lived in a series of houses, always near the Nation of Islam's Mosque Maryam or Elijah Muhammad's residence. He stayed in Chicago for about 12 years.  Only a few journalists (most notably Howard Cosell) accepted the new name at that time. Ali later announced: "Cassius Clay is my slave name." Not afraid to antagonize the white establishment, Ali stated, "I am America. I am the part you won't recognize. But get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me." Ali's friendship with Malcolm X ended as Malcolm split with the Nation of Islam a couple of weeks after Ali joined, and Ali remained with the Nation of Islam. Ali later said that turning his back on Malcolm was one of the mistakes he regretted most in his life.  Aligning himself with the Nation of Islam, its leader Elijah Muhammad, and a narrative that labeled the white race as the perpetrator of genocide against African Americans made Ali a target of public condemnation. The Nation of Islam was widely viewed by whites and some African Americans as a black separatist "hate religion" with a propensity toward violence; Ali had few qualms about using his influential voice to speak Nation of Islam doctrine. In a press conference articulating his opposition to the Vietnam War, Ali stated, "My enemy is the white people, not Viet Cong or Chinese or Japanese." In relation to integration, he said: "We who follow the teachings of Elijah Muhammad don't want to be forced to integrate. Integration is wrong. We don't want to live with the white man; that's all".  Writer Jerry Izenberg once noted that, "the Nation became Ali's family and Elijah Muhammad became his father. But there is an irony to the fact that while the Nation branded white people as devils, Ali had more white colleagues than most African American people did at that time in America, and continued to have them throughout his career."

Did he decide to join with them?
He continued to attend meetings, although keeping his involvement hidden from the public.