Someone who hangs around popular or famous people to gain their fans and friends.
The Coat-Tail rider will do anything to please whoever they are clinging to to remain in their spot light.
When the person they're coat tail riding isn't popular anymore, they move on to someone else.
They can do this in person, on twitter or any social networking site.
Girl 1- "Did you see Demi Lovato and Miley Cyrus hanging out recently? I thought Demi's bff was Selena."
Girl 2- "Well that's because She's a coat-tail rider. She hangs around Miley and Jonas because she'll get more offers if she's linked to them."
"That girl used to work at a grocery store, but she became this guys biggest coat-tail rider and earned a job as an assistant to his fashion designer sister."