Behavior is expected to be simillar to 'make proxy' on linked groups, it basically allows you to select which object in the group will be to 'root' override (usually, the armature), checks which other objects needs to be overridden as well, overrides the group itself too, and instantiates the group and the root overridden object. It seems to be working, though handling of armature deformation is kind of totally broken in blender2.8 currently (modifiers...). ;)