Return Value

Symbol Layer Drawing, SLD, can be added to a group layer to control all the drawing of its child feature layers collectively. Child feature layers with unclassed symbols, proportional symbols, dictionary symbology, or for polygon feature layers with graduated symbols cannot participate in SLD. The group layer must have at least one child feature layer that can support SLD or CanAddSymbolLayerDrawing() will return false. CanAddSymbolLayerDrawing() will also return false if the input group layer already has SLD added (unless the SLD is invalid) or if the group layer is a child of a parent group layer that has SLD added. When SLD has been established on a parent group layer, it cannot be set for individual child group layers nor other feature layers within the group; all SLD control is set through the parent group layer.
SLD is not available for 3D layers. It can only be set on group layers in a map or in the 2D Layers category of a scene.

Reference

GroupLayer Class
GroupLayer Members