Yes, you cannot directly stretch a group. However, you can select multiple (non-group) entities and stretch them.
The easiest is to press "Ctrl+A" (Select All), which will select all visible entities (excluding the groups), and then stretch those.
You can also use the Python API.
import XCoreMath
import XCoreModeling
Vec3 = XCoreModeling.Vec3
scaling0 = XCoreMath.Scaling(Vec3(2.0))
scaling1 = XCoreMath.Scaling(Vec3(1.2, 0.9, 2.0))
scaling2 = XCoreMath.Scaling(Vec3(1.2, 0.9, 2.0), origin=Vec3(120, 10, 34))
entities = XCoreModeling.GetActiveModel().SelectedEntities
for e in entities:
e.ApplyTransform(scaling0)
the different scalings are
uniform scaling
non-uniform scaling
non-uniform scaling wrt to the (non-zero) origin=(120, 10, 34)
https://youtu.be/XCOM6uBFBRM
