There can’t be any distinction between membership and communion since one of the four Marks of the Church is Her Unity.
But there are three elements of unity:
1) Faith,
2) Governance,
3) Sacraments.
If a man be lacking in any of the three, he cannot be a member.
But what if he has one or two of the three?
Many of the saints, Fathers, and Doctors (eg., St. Thomas Aquinas, St. Augustine, St. Bellarmine et al) spoke of such being in communion with the Church, even as they remained outside it).
This implies to me that “communion” and “membership” could be two different things, which are incessantly conflated.