The Steam Workshop can be used to share vehicles and other creations between players.
When you load a vehicle into the workbench editor you can choose "upload to workshop" in the top bar. Give your creation a name and select tags for it so users can find it easy. After you uploaded it go to your steam bibiothecary, select "Stormworks: Build and Rescue" in the left list. On the right side you can now see the normal information about the game. There should also be a button "Workshop". Hit that. On the right side of the page that opens you can see your profile picture. Bellow you can click the dropdown and choose "my workshop object". Now you can see your newly added vehicle.
You are now able to adjust the description, add screenshots and videos.
Important: Do not change the automatically generated preview image of your vehicle by editing the png in the %appdata% folder. That is illegal and can lead to a ban!