If you want to add single files to the library, rather than folders...
1. In LW, click on File > Add file to library.
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2. Navigate to the folder which contains the file(s) you want to add. Double click on the folder. Highlight the file that you want to add. Click on the Add File button.
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If you want to share the file(s), see post #3 for info.

If you want to remove a single file from your library, right click on the file > remove from library. The file will be removed from your library but it will still be in the folder where it was originally saved on your computer. To remove multiple files at once, click on the first file that you want to delete & holding down the control (ctrl) key, click on other files that you want to remove. Right click > remove from library & any highlighted files will be removed.