Any registered user can send & receive Sweatcoins. To transfer Sweatcoins, go to your Balance and select 'Transfer' and fill in the person’s username (this is not simply their name so make sure you have the right username i.e. johndoe342345. You can also make direct transfers from your friend's profile page - just click the "Send" button.
Always triple check that you have the right username of the person you're trying to send to. Very soon the functionality allowing Sweatcoin to reverse transactions will cease.
When this kicks in, if you accidentally send Sweatcoins to the wrong user, they would be lost to you... forever. Unless of course, the other user was friendly enough to send them back!