The Assembly claimed that it had no such thing as "membership". You didn't "join" the Assembly as you would a normal church. Instead, you "came into fellowship", or better yet, you "made a commitment to fellowship." One reason for making this distinction was that coming into the Assembly had much more significance than just joining a church which you might later decide to leave for another church. Making a commitment to fellowship in one of the Assemblies brought you "under the covering", a term for God's special protection, which you would never want to leave for any reason. A former Worker contributed this diagram that was used to teach the concept of "coming into fellowship in the Assembly".
The Bible passages cited here all use the word "fellowship", but they are not speaking about joining oneself to a local gathering of believers.
Another point to note is that the title of this diagram is "Called into the Fellowship" - insinuating that Assembly fellowship is singular and unique. The diagram itself show how this is so. If I live in a properly authorized, properly governed Home (note the capitalization here), which is part of a properly ordered Gathering (note again the capitalization), which is under the direction of The Work (that is specifically George Geftakys' Work), then as a believer I will be under the covering of God in Christ. This diagram is not acknowledging just any home or any gathering of believers, only those under George's ministry. There only is your safety as a Christian from the surrounding world of rebels, a very dangerous outer world. These definitions point to the correct answers to the questions posed in the right column.