Each year on Holy Saturday during the Easter Vigil,  thousands are baptized into the Catholic Church in the United States. Parishes welcome these new Catholics through the Rite of Christian Initiation of Adults.

What is meant by coming into full communion with the Church?

Coming into full communion with the Catholic Church describes the process for entrance into the Catholic Church for already baptized Christians. In most cases, these individuals make a profession of faith but are not baptized again. To prepare for this reception, the people, who are called Candidates, usually participate in a formation program to help them understand and experience the teachings and practices of the Catholic Church. Although some preparation may be with Catechumens preparing for baptism, the preparation for Candidates is different since they have already been baptized and committed to Jesus Christ, and many have also been active members of other Christian communities. The Candidates may be received into the Catholic Church at the Easter Vigil or at another Sunday during the year depending on pastoral circumstances and readiness of the Candidate.
(usccb.org)