How do I combine two node trees? I have a node tree that gives me a background with a colour of my choice instead of the default black.

I also have a node tree that controls mist in my scene.

No problem with either of these. They both work as I wish. My question is how do I combine two node trees? In other words, in my example, how do I have a coloured 'sky' and mist?
If I have nodeTrees that I would like to share between Files or anywhere -> I group all the nodes I need - give it a proper name and insert the grouped-Tree anywhere else from shift-A menu...or append it in some other file.