Are players required to participate in a Challenge?
When a player creates a Challenge, they may invite other players to participate. Those players are free to ignore the invitation, and thus decline the challenge (without ever having to overtly decline the invitation).
Players in a Team can be placed into a Team Challenge by the Administrator as a member of a Squad. In those instances, players don’t have the opportunity to decline via an interface, but they should have direct real-world access to their Administrator to ask to be excluded or for any other change in participation.