I've found some neat combinations of nodes to create certain effects, so I'd like to make them a group and then have that node available on my list of available nodes, even when I reload the startup file. How would I do that?
You can save the startup .blend by clicking File > Defaults > Save Startup File. This will save everything (except for user preferences) to be loaded as the default state when Blender starts up. That includes all geometry, textures, brushes, node groups, UI layouts, the 3D view orientation, etc.
I like to save my node groups in a separate "library" file and link/append them in, that way I don't accidentally lose them if I overwrite my startup file on accident (I've done that before..)
How to embed a node group in the startup file
If you haven't already, make a node group by selecting the nodes and pressing CtrlG (AltG to ungroup).
Save it (CtrlS)
Open your startup file (CtrlN)
Press File > Link or File > Append. In the file manager which appears, navigate inside the file with the node group, select NodeTree and then select the node group.
Linking means the node group is only stored in the "library file" and if you make any changes to it, those changes will carry over to your startup file (also, if the library file is deleted or moved, the node group will be missing in the startup file).
Appending copies the node group so it will always work.
In the node editor, add the node group (ShiftA> Groups > group name) and press the
Ficon (F for Fake User) on the node to save the data block even if it has no users. This will ensure the node group is saved even if it is not used in any materials/node trees in that file.
Delete the group node (Del). Provided you enabled Fake User by clicking the F icon, it will still be in the add menu even after reloading the file. Rearrange the UI how you want it to appear when you start blender.
Save the startup .blend (File > Defaults > Save Startup File).
This procedure should be the same for all types of nodes (Composite, shader, etc.)
I wouldn't recommend to add everything to the Startup File. You could also create a node library (.blend file) to hold all your nodes and link and/or append groups from the library to your current project.
Accessible from the Info Header
File Link Ctrl-Alt-O
File Append Shift-F1
A nice example library: bwide-nodepack-for-blender
You find the nodes within the .blend file in the folder NodeTree:
- Video tutorial on Creating a Reusable Node Library