Either they entered your username in the registration screen.
Or they followed a link in the "send to friend" email, and as above that includes your userid.
@Velocio started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Either they entered your username in the registration screen.
Or they followed a link in the "send to friend" email, and as above that includes your userid.