Multiple Layers of Fellowship

1 Corinthians 1:2 KJV

Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours:

This one passage may reflect the multiple layers of fellowship believers have with one another

Unto the church

This is unquestionably a reference to a local congregation as it is at Corinth and the very term "church" demands it be an assembly. An assembly is always made up of qualified membership. It is not merely a gathering or mass of people, it has order and purpose.

To them that are sanctified

This could be viewed as a reinforcement of those who are qualified to be members: sanctified and called. It could also reflect that within this congregation were people who have been called of God to salvation but who have not yet united to the membership of this church through baptism. Any sound church will have those it has reached but has incorporated into the church yet. 

With all that in every place

This phrase acknowledges the existence of and relationship to all believers everywhere. It is not a church relationship. It is not the same as a purposeful and organized assembly; those ought to exist in each locality, but it is a communion between brethren. Each local church is it own organism with Christ as its head. Each local church is related to and in some way in fellowship with all others in everyplace so long as they in fact call upon the name of Christ.