‘ And they came to the children of Reuben, and to the children of Gad, and to the half tribe of Manasseh, to the land of Gilead, and they spoke with them, saying, “Thus says the whole congregation of YHWH, ‘What trespass is this that you have committed against the God of Israel to turn this day from following YHWH, in that you have built yourselves an altar to rebel this day against YHWH?' ” '
Note that the congregation of Israel has become ‘the congregation of YHWH'. The approach was in the name of YHWH because the alleged offence was a religious one. We can compare how the phrase was applied to the sin of Peor (Numbers 31:16). Their view was that the building of the altar was in order to rival the central sanctuary, and set up a rival place for worship. It was rebellion against YHWH Himself, the God of Israel. The point was that it was not a place in which YHWH had ‘recorded His name' (Exodus 20:24), for there such an altar would have been permissible.
“The congregation of YHWH” is among other things the worshipping community at the Tabernacle (Deuteronomy 21:1-3; Deuteronomy 21:8. See also Numbers 16:3; Numbers 20:4; Numbers 27:17).