A group in the sidebar denoted by a folder icon is just that - a group. This is organizational concept.
You create it by selecting several shapes and grouping them by clicking Group icon. Since this is organizational concept, you can't apply shape's action to it, like masking, shadows, etc.
A group in the sidebar denoted by a shape icon is just that - a ...
For Sketch 44+, this becomes quite simple.
Create an auto-width text layer
Place an object (like a rectangle) to the right of that layer
Select both the text layer and the object
From the layers panel choose "Create Symbol"
From the create symbol dialog, choose "↦Left to Right Layout" in the dropdown underneath the symbol name input