From the Council of Florence (1439):
“Three of the sacraments, namely baptism, confirmation and orders, imprint indelibly on the soul a character, that is a kind of stamp which distinguishes it from the rest. Hence they are not repeated in the same person. The other four, however, do not imprint a character and can be repeated. Holy baptism holds the first place among all the sacraments, for it is the gate of the spiritual life; through it we become members of Christ and of the body of the church.”
Likewise, Pope St. Stephen I (in the third century), affirming the constant teaching of the Church, condemned the rebaptism of non-Catholics who were already Baptized.
So, yes, anyone who has been validly Baptized, and is not mortally guilty of heresy, schism, or apostacy is a member of the Church. But, this communion is imperfect because it Christ wills that we be of one mind professing the same faith united under one shepherd.
Also, since we cannot judge whether or not someone is mortally guilty of the above sins, just like any other grave sin we should do what we can to persuade others to abandon their possibly damning errors and come into full Communion with the Church.