For this issue specifically, Clint Black from Pinnacle has clarified on the official forums that all characters acting on a single initiative—whether Wild Card alone, Wild Card with allies, a single extra, or a group of extras—act on a Joker and receive the benefits it provides (i.e. acting any time and +2 to Trait and damage rolls), and that it is no different for player characters or GM characters.
Generally when the rules refer to all participants at the table except for a GM, they use the term "player characters." For instance, the "Joker's Wild" setting rule specifically uses that term when talking about Jokers:
This is a great rule to add to your game if you’re the kind of Game
Master who’s a little stingy in awarding Bennies. It slightly ups the
heroics and is a lot of fun for the group.
When a player character
draws a Joker during combat, he receives his normal +2 bonus to Trait
and damage rolls. In addition, all player characters receive a Benny!
However, Pinnacle seems to use the term "players" to indicate all participants at the table. For instance, the rules on Bennies, Shaken, Tests of Will, rate of fire, and multiple actions all cite "players", and you'd have a crippled game if all of those rules did not apply to the GM. Some times, Pinnacle uses the phrase "players and the GM", but the way they use it seems to be synonymous with the way that they use the term "players" elsewhere.