Members are Club Penguin players who pay to access features not available to non members. They can acquire clothes, play all levels of mini games, go to special rooms during parties, adopt up to twenty puffles of any species (whereas non-members can only get two puffles which are red or blue), buy puffle clothes, get igloo upgrades & furniture, and get a badge on their player card to prove their membership. They can also open their igloos to other players.
Though members are not necessarily a social group, they have a reputation on Club Penguin. Some people think members are full of themselves and that they look down on non-members. Though this is the case with some, most members are rather nice to non-members, especially if they're older or have been one themselves.
Members are higher in concentration on servers like Abominable or Zipline, because these are usually full to players without membership.