The church of the Firstborn is not a physical church or group. It doesn’t have leadership and all lines of authority are 1 step to God. This was the reason the D&C was given as stated in the Introduction chapter.
D&C 1:18-22
18 And also gave commandments to others, that they should proclaim these things unto the world; and all this that it might be fulfilled, which was written by the prophets—
19 The weak things of the world shall come forth and break down the mighty and strong ones, that man should not counsel his fellow man, neither trust in the arm of flesh—
20 But that every man might speak in the name of God the Lord, even the Savior of the world;
21 That faith also might increase in the earth;
22 That mine everlasting covenant might be established;
Every member of the Church of the Firstborn has a direct connection with God and angels, and doesn’t need middle men. (This process is learned.)
When men organize, they have regular meetings, Top down or bottom up leadership, processes & Protocols all of which results in the hands of men creating, building and directing a particular work. The Church of the Firstborn is not organized by the hands of men, but by the spirit, angels or even Christ. This is the rock cut without the hands of men. Two people may receive different instructions which in the end result in unorganized yet collaborative work. For example, see Alma 10:7-10.
Being in the Church of the Firstborn does not require membership in the LDS Church although most come to it through that path.