All the details are here. Magnus Carlsen declined his invitation this year, and Vladimir Kramnik did too (as usual). Six of the Candidates did sign up, however: Shakhriyar Mamedyarov, Fabiano Caruana, Levon Aronian, Wesley So, Alexander Grischuk, and Sergey Karjakin; additionally, Maxime Vachier-Lagrave, Viswanathan Anand, and Hikaru Nakamura are joining the party. Kramnik will be a wildcard in Paris, and Anish Giri will participate as a wildcard in Leuven. As for the two St. Louis events, the wildcard remains open. Maybe someone with the initials G.K. will perform an encore?