Any registered user can send & receive sweatcoins. To transfer sweatcoins, go to your Wallet 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 as you can't reverse transactions!