As the plugin API document says, the group api allows to create the new group while keeping all the grouped layers in the same absolute x/y locations. However, it dosen’t work as expected in my case:
// the absoluteNode has the layoutPositioning "ABSOLUTE"
figma.group(pabsoluteNode], absoluteNode.parent);
// the x/y locations of absoluteNode are changed, and the new group has the layoutPositioning "AUTO"
Also, I’ve double checked the “Group selection” menu in Figma editor and it works all right. It seems that there is bug in group API. Hope it to be fixed.