Isaiah now tells the people of Judah that their human, worldly plans for their salvation will come to naught. When he says: “associate yourselves, O ye people, and ye shall be broken in pieces,” Jehovah has Isaiah tell Judah that they may attempt to make alliances, typically with Egypt, but that alliance will avail them nothing.
When he says “take counsel together, and it shall come to naught,” he means that whatever plans they make will be futile. Since they have turned against their God, Jehovah is not with them, and without Jehovah’s support, they will fall.