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 and the user preferences by pressing CtrlU (note that in 2.66 the startup .blend and user preferences were split. See this post for more detail).
Also note that the startup .blend contains everything a normal blendfile does, so any geometry, textures, brushes, node groups, UI layouts, view orientations, etc. will be saved and loaded by default when you start blender.
To create and save a node group:
Note that this procedure is the same for all types of nodes (Composite, shader, etc.)
Recovering the default startup
You can reload the default settings and startup.blend by pressing File > Load factory settings:
Note that this only loads the defaults, it does not reset the default .blend. To overwrite your startup .blend with the defaults, load the factory settings and then press CtrlU.
In case you want to back up (or delete, replace, etc.) your startup.blend, navigate to the
To find the user path for your OS, see the wiki
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
A nice example library: bwide-nodepack-for-blender
You find the nodes within the .blend file in the folder NodeTree: