As LuckGrib downloads or imports GRIB files, it stores them in a private area, dedicated to application specific data as defined by the Apple developer guidelines. This area is considered part of the internal database for the program, and you are discouraged from modifying anything in there.
If you would like to make a GRIB file available to other programs, or would like to email it to someone, you will need to export it from LuckGrib.
On the Mac, this can be accomplished by selecting the file you want to export, and then invoking the Export File… menu item from the File menu.
On iOS, you can export a file by swiping left on the file in the Weather Files list, and then sharing it. You can share the file via email, AirDrop, export to another application such as Files, along with other possibilities