Answer: You can't use the .sket file directly. You can send a JPG version of the work to anyone through supported share options.
Pseudo (Quick) Answer: If you want to work with the vector paths within the Sketch (.sket) project you'll need an Adobe CC Subscription ~
1. Go into the specific illustration you want to send (click on it as if to edit it in the app);
2. Send (Share) to Illustrator CC from the mobile app. (to the same CC account that you're logged into your iPad with);
3. Within Illustrator CC, Choose from toolbar options OBJECT > LINE & SKETCH ART > EXPAND TO PATHS
NOTE: this doesn't apply the brush work. It converts your artwork to simple paths and strokes, changing your work to something that doesn't resemble the .sket project. Fortunately, it doesn't save over the original .sket file.
Thanks for the idea. Unfortunately that process kills all the texture of a brush or chalk line from Adobe Sketch and makes it into a flat, boring stroke. I guess some artists use non-vector objects in Illustrator. I only use Illustrator for vector based work so this conversion is pretty mush useless for me.
The only way I can get around it is by rasterizing the .sket file then using Image Trace.
Maybe you have a better idea.
Unfortunately the option to expand to paths within Illustrator does not seem to work anymore since the latest version of Adobe Sketch (3.4).
Sending the sketch artwork to Illustrator from the mobile app now sends a layered .psd file to Illustrator and it used to be a .sket file.
Opening the sketch artwork from the 'Mobile Creations' opens it as a .png file
I hope that it will be available again in the next version of Adobe Sketch.