The first occurrence.
Speak to all the congregation of Israel, saying, In the tenth day of this month they shall take to them every man a lamb, according to the house of their fathers, a lamb for an house: Exo 12:3
The primitive root.
Strong’s H5712, edah, a concrete noun meaning, “a fixed gathering,” from Strong’s H5707 ed, a concrete noun meaning “a witness,” contracted from Strong’s H5749 עוד uwd, a primitive root meaning, “to repeat, to bear witness.” The 3-letter root is ayin + vav + dalet.
ayin ע = the eye, thus watch, know, shade
vav ו = the tent peg, thus add, secure, hook
dalet ד = the door, thus enter, move, hang
All judgments and binding contractual business in the patriarchal era was conducted either at the door of the tribal authority, who was the patriarch of the tribe, where he sat in the shade to decide family matters:
Then YHVH appeared to him by the terebinth trees of Mamre, as he was sitting in the tent door in the heat of the day. So he lifted his eyes and looked, and behold, three men were standing by him; and when he saw them, he ran from the tent door to meet them, and bowed himself to the ground. Gen 18:1-2
Or at the gate of the city, where the elders congregated:
Then Abraham stood up and bowed himself to the people of the land, the sons of Heth. And he spoke with them, saying, “If it is your wish that I bury my dead out of my sight, hear me, and meet with Ephron the son of Zohar for me, that he may give me the cave of Machpelah which he has, which is at the end of his field. Let him give it to me at the full price, as property for a burial place among you.” Now Ephron dwelt among the sons of Heth; and Ephron the Hittite answered Abraham in the presence of the sons of Heth, all who entered at the gate of his city, saying, … Gen 23:7-10
The story: What one has seen or witnessed (ayin) verified or sworn (vav, with the idea of securing the veracity of the claim by oath) at the patriarch’s door (dalet, or at the city gate, which is the door to the city).
The relationship of the root, to bear witness, to the noun, congregation, has to be that when Israel congregates together they are bearing witness to the one true God, YHVH.