All known references to the Law of Assembly by Alice A. Bailey (AAB) in her voluminous writings and those given to her by Djwhal Khul (DK). Links on these pages of The Laws of Alice A. Bailey are to the online books at the Lucis Trust website.
The Law of Assembly appears in Discipleship in the New Age, Vol. II and nowhere else that I can find.
What is a law?
… a law is but the spiritual impulse, incentive and life manifestation of that Being in which he lives and moves. A Treatise on White Magic , INTRODUCTORY REMARKS
A law is an expression or manifestation of force applied, under the power of thought, by a thinker or group of thinkers. Discipleship in the New Age, Vol. I , SECTION ONE – DISCIPLESHIP IN THE NEW AGE – Part 2
… a law is in reality the effect of the life of a greater entity as it encloses a lesser within its living processes.
Esoteric Healing , Part Three – The Fundamental Laws of Healing
… a law is but the effect of the continued intelligent activity of the Life aspect as it works in conjunction with matter. Esoteric Psychology – Volume I , 5. The Rays in Cyclic Manifestation
A law is only an expression or manifestation of force, applied under the power of thought by a thinker or group of thinkers. Esoteric Psychology – Volume II , II. The Seven Laws of Soul or Group Life – Part 5
A law presupposes a superior being who, gifted with purpose, and aided by intelligence, is so coordinating his forces that a plan is being sequentially and steadily matured. A Treatise on White Magic
Quotes I found below:
This point of revelation carries the initiate to one of the highest points of contemplation; we are here concerned with his sudden apprehension—at the fourth Initiation of Renunciation—of another phase of the divine, conditioning Will. He has begun to recognise and to interpret—even though in an elementary fashion compared to what he will recognise at the ninth initiation—the destroying aspect of the Will as it expresses itself through the Law of Sacrifice. Now, for the first time (as a result of destruction), he can begin to grasp the essential building aspect of this same Will and to appreciate his future function as a creative Builder. The building here to be carried forward, I would remind you, is not the building which is distinctive of the second divine aspect—that of Love-Wisdom. It is strictly connected with that of the first aspect of Purpose, Power or Will; it deals with the processes which precede the actual creative building, the drawing up of the blueprints (if I may use such a term) “within the confines of Shamballa,” where high spiritual Beings must lay Their plans. This is a different process to the creative building process, and is related to a mysterious undertaking which is carried forward under the “Law of Assembly.”
The energy which is employed and with which the initiate at this time makes his first and very temporary contact is that which the planetary Logos employs as He gathers into His ring-pass-not that extra-planetary substance which must progressively be used, as the world of being and of forms makes progress under the Law of Evolution. It must be remembered, for instance, that the planetary forms are now composed of a much higher substance than in primeval days. It must also be remembered that a great process of elimination is forever going on, accompanied by a paralleling process of substitution. That which is rejected and which is occultly thrown out as no longer serving the purpose of the planetary Logos is replaced by that which “will measure up to the purpose of the Father.” It is this concept which has been travestied and distorted by the Christian teaching anent the “vicarious atonement.” This grew out of a basic confusion between the Law of Sacrifice and the Law of Assembly, and took place when any understanding of the nature of the Father remained a supreme mystery; the whole process was then interpreted in terms of the first and highest Aspect of the essential Trinity, of which man knew nothing, instead of in terms of the second Aspect, the building, magnetic aspect of Love. We are—if it could be but realised—in process of re-interpreting and re-arranging what can be called “the doctrinal structure underlying the relation between knowledge and wisdom.” This involves the destruction of old concepts such as the trinity of manifestation, and the assembly of those new and more correct ideas which must inevitably be substituted for the old, as the unfoldment of the first aspect is presented to the initiate upon the Path. This, through certain later activities, will gradually seep downwards into the consciousness of humanity, and the new world religion will be founded upon a deeper spiritual perception of the Father or Life Aspect, in place of the rapidly crystallising vision of the Son or consciousness aspect.
The process in which the initiate is now permitted to share during and after the fourth Initiation of Renunciation (which is presented to him in a successive series of points of revelation) is made possible by his submission to the Law of Sacrifice. This, in its turn, brings his consciousness under the influence—in developing stages—of the Law of Assembly. This is a law to which I have not yet made reference in any of my previous instructions. The initiate can now—through appreciated and comprehended revelation—add to his work with the Plan and its building technique, a capacity to work with divine Purpose and with those substances (etheric in nature) and those energies which permeate the cosmic etheric body (the four higher planes of our cosmic physical plane) under a law which is set in motion by the Father, but which makes the building activity of the Son possible, so that it is utterly in line with divine Purpose.
There is, first of all, the recognition of a great procedure which must, under the Law of Assembly, be responsible for the bringing in of extra-planetary energies as the redemption of the worlds of being and of form is carried forward by the second divine aspect. Of this procedure I can tell you naught. The place or the location of the procedure of assembly is made clear. It is in the Council Chamber of Shamballa. I would, however, brother of mine, call your attention to the fact that Shamballa is simply a word conveying the idea of a vast focal point of energies which are assembled and brought together by the planetary Logos in order to create a manifestation adequate to His unfolding intention and planetary service. The objective is equally clear; it is the assembly of that which has in it energies of redemption. Just as the Science of Redemption, under the hierarchical Plan, is the technique employed to redeem the world of appearances, so—on a much higher level—the Law of Assembly  produces the Science of Energy which is the redemptive process (in a sense which you cannot possibly understand) influencing the work of those who labour for redemption in the world of meaning.
You are welcome to post your comments and understanding of this law.