Grier, a Vine and YouTube personality, and Espinoza, a triple Crown-winning jockey, have both joined the Dancing with the Stars cast.
Grier, 15, made the announcement of his participation on Twitter Thursday, writing to his three million followers that he's "not good at keeping secrets."
"Right now it's 4 in the morning in L.A., and I was going to announce my big announcement in seven days, but I just got word that I could announce it now, so why not?" a sleepy Grier said in a video posted to the Challenged app.
"It's going to be really cool and an amazing journey to be on season 21 of Dancing with the Stars, and I'm super excited, and I want you guys to be a big part of it and support and vote for us, and it's going to be a great time."
As for Espinoza, news of his deal with ABC circulated Tuesday when he admitted on SportsNation he'll be a part of the show but can't dance and has no rhythm, according toEntertainment Weekly.