System Information
Operating system: Windows 10.0.19042 Build 19042
Graphics card: AMD Radeon RX570

Blender Version
Broken: 2.93.0 Alpha (hash: 3494946560c2)

Short description of error
nodeitems_builtins contains a function used for listing node groups which filters out "hidden" groups - denoted by the name starting with a period.

https://github.com/blender/blender/blob/2f7258d618a2a38fc982a9e0d0d7dd3cbc95ab3a/release/scripts/startup/nodeitems_builtins.py#L118

This prevents those groups showing up in the node editor when adding a group node, and is very useful for custom node groups, especially in the context of auto generation by an addon via ShaderNodeCustomGroup. However, there exists another UI for selecting node groups which does not do the same filtering: in the material properties panel by clicking "Surface". Custom node groups which should be hidden clutter the box.

The guilty function appears to be uiTemplateNodeView.

Exact steps for others to reproduce the error

1. Add a mesh and node based material for that mesh
2. Edit the nodes and create a node group
3. See that the node group shows up in the Add > Group menu
4. Rename the node group .HiddenNodeGroup or similar
5. See that the node group is hidden in the Add > Group menu [first image]
6. See that the node group still shows up in the Properties > Material > Surface dropdown [second image]