Gettin position of an object insede a group after rotate group

Questions
1

I’m trying to connect a line between Vector A and Vector B, Vector B (position relative to the group) belong to a group and the other is outside, if I rotate the group, how can I get the Vector B position relative to vector A?

2

If you mean by “group” a hierarchy of Object3D then you get the world position by

const positionB = new THREE.Vector3;
positionB.setFromMatrixPosition( childObject.matrixWorld );
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