From Bulbapedia, the community-driven Pokémon encyclopedia.
A Club Member is a normal Trainer who usually duels anyone inside a Club lead by a Club Master.
Usually when a Club Member is defeated by the player, they award the player a booster pack or two.
Some Club Masters requires the player to defeat their Club Members first before battling them.
They are the Pokémon Trading Card Game counterparts to the normal Trainers who usually hang out in a Gym.