Users need to accept you as a friend before you can send them Snapchats

Unless it has changed recently, I think anyone can receive Snapchats until disabled. At least, that's how it was when I opened an account a few weeks back. That was discovered when I received an unsolicited snapchat from a name I didn't recognize (I have no friends on the service aside from the default teamsnapchat).

Rather than risk viewing something regrettable, I deleted the "message" and quickly found the setting to disallow unknown users from messaging me. :)