There is no way in default blender to directly export/import node groups into Blender. There are two other methods you can try though.
Back in 2014, Luca Rood made an add-on that allowed users to export node groups to .ngr files, and later import them into other projects. This is for a much older version on Blender though, so I can’t say whether or not it will work for sure today. If you want to give it a shot, here’s the Blender Artists page: https://blenderartists.org/forum/showthread.php?346359-Addon-Export-node-groups
The better method, in my opinion at least, is to just append them into your new files. Save all the node groups you want to keep into one Blender file, then append them into your other projects. This can be done by clicking File>Append, navigating to the .blend file with your node groups, opening the node tree folder within the .blend file, and selecting whatever node group you may need.
Hope this helps!
