The first chapter of Watchman Nee’s book Assembling Together (chapter 14 of the Basic Lessons series) is called “Joining the Church”. This is a great chapter with which to begin to understand Nee’s ecclesiology.
The phrase “joining the church” is quite interesting. To Nee, this means something completely different to how I’ve seen this phrase used in contemporary churches in the United States. I think even Nee understands how this phrase is normally used. He says, “We do not like the phrase ‘joining the church,’ but use it temporarily to make the issue clear.”  So, what does Nee mean by “joining the church”? He first explains how believers immediately become part of God’s family upon salvation. He then specifies exactly what he means by “joining the church”:
A Christian therefore must join the church. Now this term, “joining the church,” is not a scriptural one. It is borrowed from the world. What we really mean is that no one can be a private Christian. He must be joined to all the children of God. For this reason, he needs to join the church. He cannot claim to be a believer all by himself. He is a Christian only by being subordinate to the others. 
Never once in the Bible do we find the phrase “join the church.” It cannot be found in Acts nor is it seen in the epistles. Why? Because no one can join the church… Rather, we are already in the church and therefore are joined to one another. 
When, by the mercy of God, a man is convicted of his sin and through the precious blood is redeemed and forgiven and receives new life, he is not only regenerated through resurrection life but is also put into the church by the power of God. It is God who has put him in; thus he already is in the church. 
Then why do we persuade you to join the church? We are only borrowing this term for the sake of convenience. You who have believed in the Lord are already in the church, but your brothers and sisters in the church may not know you. 
At this point, Nee remains close to Scripture. He is correct that “joining the church” is not a scriptural phrase, and is never commanded or exhorted in Scripture. Instead, we become part of the church when we are “born again” into the family of God. It is true that we may still need to seek out brothers and sisters with which to fellowship, but that is not the same as “joining the church”. Of course, the best place for a new believer to begin to find fellowship with other brothers and sisters is with the person or people who made the gospel available to him or her.
Next, Nee answers the question: which church should I join? Most believers today would probably disagree with his answer. First, Nee explains the rise of different churches and denominations based on time, area, human personalities, or a particular emphasis on one aspect of truth. He then says that all believers in a city form a city-church, and that is the church that a new believer should become part of. In fact, he argues that the only valid biblical definition for “church” (singular) is the city-wide church:
The Bible permits the church to be divided solely on the ground of locality… The smallest church takes a locality as its unity; so does the biggest church. Anything smaller than a locality may not be considered a church, nor can it be so recognized if it is bigger than a locality. 
This statement is problematic. Nee examines several passages to demonstrate that the singular “church” is used to represent all the believers in a given city. I do not have a problem with this analysis, except I think he left out a few key passages of Scripture. It is not true that the singular “church” is always used to represent all the believers in a city and that the singular “church” is only used to represent all the believers in a city. Here are a couple of passages that use the singular word for “church”, but may not represent all the believers in a city or the believers of only one city:
But Saul was ravaging the church, and entering house after house, he dragged off men and women and committed them to prison. (Acts 8:3 ESV)
Greet Prisca and Aquila, my fellow workers in Christ Jesus, who risked their necks for my life, to whom not only I give thanks but all the churches of the Gentiles give thanks as well. Greet also the church in their house. Greet my beloved Epaenetus, who was the first convert to Christ in Asia. (Romans 16:3-5 ESV)
I should also mention that in Acts 9:31, some manuscripts have the singular “church” (while others have the plural “churches”) for the believers in the regions of “Judea, Galilee, and Samaria”. There are also other passages that mention the “church” in someone’s house which may or may not be the entire church of a city.
So, I do agree with Nee that Scripture describes all the believers in a certain city as a “church” (singular). However, it appears that there may be smaller groups within that city-church that are nevertheless called “church” (singular). Similarly, in Acts 8:3, it appears that Saul is persecuting believers over a larger area than a city, but Luke still considers Paul to be persecuting the “church” (singular). The usage of the word “church” is more complicated that Nee makes it out to be.
There is one other point (and a major point, I think) with which I disagree with Nee. He claims that individuals are not the dwelling place of God; only the church is God’s habitation:
In the past God dwelt in a magnificent house, the temple of Solomon. Now He dwells in the church, for today the church is God’s habitation. We, the many, are joined together to be God’s habitation. As individuals, though, we are not so. It takes many of God’s children to be the house of God in the Spirit. 
Unfortunately, I do not think that Nee considered enough scriptural evidence. It is true that most of the references to the Spirit dwelling within beleivers occurs in the plural. But, of course, most of Scripture was written to communities of believers to be read to the entire community. It is also true that the Spirit dwells with the community; however, just as Solomon’s temple could not contain God, the community alone does not contain God’s Spirit. There are plenty of references to individual believers being filled with the Spirit of God (i.e. Acts 6:3, 9-10).
Besides these two points of disagreement, this is an excellent chapter. Nee encourages all believers to find other believers with which to fellowship. He especially exhorts new believers that they should not try to live in isolation.
I usually find the last paragraph of one of Nee’s chapters to be very helpful. Sometimes, even when I do not agree with Nee’s arguments, I agree completely with his conclusion in the last paragraph. I agree with much of this first chapter, and I also agree with his last paragraph:
You who are already in Christ should learn to seek the fellowship of the children of God. With this fellowship of the body you may serve God well. If you as young believers can see this light, you will move a step forward in your spiritual path. Thank God for his mercy. 
The next chapter in this book is called “Laying On of Hands.”
Review of Watchman Nee’s Assembling Together Series:
1: Chapter 1 – Joining the Church
2: Chapter 2 – Laying on of Hands
3: Chapter 3 – Assembling Together
4: Chapter 4 – Various Meetings
5: Chapters 5 & 6 – The Lord’s Day and Hymn Singing
6: Chapters 7 & 8 – Praise and The Breaking of Bread