If I haven't imported my RAW files into Lightroom with the sidecar files attached, how can I save/export the file so that another person can open the file and not have to re-do all my work?
Probably one of the easier ways is to convert the file to DNG, which will allow the changes to be embedded in the file. Adobe has a writeup on how to do it. It does mean, however, that the recipient must have software that can handle DNG, but there are both commercial and free platforms that can do it.
The latter 2 answers suggested to utilize DNG.
The only other method would be to utilize sidecar files (.xml) files that lightroom provides.
Adobe's Julianne Kost cover's how to save your changes in lightroom:
I've had recently had a lightroom catalog die on me. I had to create a new lightroom catalog. The sidecar files don't save your information about flagging or picks. I had to redo that. Brushes, exposure settings, etc were all saved!
Heres a workaround for saving picks: Export/backup files from Lightroom including flags/stars/picks
I would say you have two options. You can either export your library to DNG files, or generate XMP sidecar files for your catalogs. Either way should be effective, and ultimately both are probably similarly compatible.
To export your images to DNG, simply go to Library mode, select the root folders of your collection, and change to grid view. Hit CRTL+A (Windows...not sure of the mac shortcut) to select all, then hit CTRL+SHIFT+E to export. You can either use the Lightroom preset "Export to DNG" and simply export...which will use the original names with new extensions, embed metadata, or you can customize and choose how you wish to name, what size preview jpeg to embed, whether to resize, etc.
Exporting an entire catalog to DNG can take a while, especially if you have configured Lightroom to embed original RAW images in the DNG's as well. A quicker alternative might be to simply enable XMP sidecar files, and generate metadata for your library. Generating XMP sidecar files is pretty easy...simply right-click any root folder in any drive in your catalog, and select "Save Metadata". Lightroom will generate XMP sidecar files for every photo you currently have included in your catalog. In my catalog, which is about 8000 photos right now, it takes a couple minutes to generate XMP sidecars for every photo...all of which are RAW.
My advice would also be to use DNG as DNG is a raw format.
However, I believe that it if you tell Lightroom to write the chances to XMP it will create the sidecar file if needed, so rather than exporting all you need to do is right click, then show in Finder (or Explorer if you're using Windows) and then find the sidecar file.