How to share your deck
From your account, click into the deck you'd like to send. Make sure the deck is toggled "public" — this means your viewer will be able to access it. Then select the "Share" button on the top right:
This should pop up a box:
You can then copy that link URL, and send it to anyone.
How to receive (copy and use) a deck
The link will take your viewer to a "sharing" preview page, where they'll be able to see your slides. There will be a button on that page that says "Copy & use this deck" (If this button says "create an account", they may need to log in or create an account first).
How it works
If they click the "copy" button, it will make a duplicate of the deck and paste that duplicate into their SlidesWith account. In other words, they're making their own copy of that deck (in whatever is its current state in your account) that will then be their deck, in their account.