UPDATE:  A number of readers object to my use of the word "member" because the links "merely" show that Obama participated in meetings, sought the nomination, and used the nomination.  I'm not striking it because I think it's an abundantly fair description.  See, e.g., this (my italics):

The NP's [NP stands for the New Party, which was formed by the Democratic Socialists of America] political strategy is to support progressive candidates in elections only if they have a concrete chance to "win". This has resulted in a winning ratio of 77 of 110 elections. Candidates must be approved via a NP political committee. Once approved, candidates must sign a contract with the NP. The contract mandates that they must have a visible and active relationship with the NP.  The political entourage included Alderman Michael Chandler, William Delgado, chief of staff for State Rep Miguel del Valle, and spokespersons for State Sen. Alice Palmer, Sonya Sanchez, chief of staff for State Sen. Jesse Garcia, who is running for State Rep in Garcia's District; and Barack Obama, chief of staff for State Sen. Alice Palmer. Obama is running for Palmer's vacant seat.

And this:

The NP's '96 Political Program has been enormously successful with 3 of 4 endorsed candidates winning electoral primaries. All four candidates attended the NP membership meeting on April 11th to express their gratitude. Danny Davis, winner in the 7th Congressional District, invited NPers to join his Campaign Steering Committee. Patricia Martin, who won the race for Judge in 7th Subcircuit Court, explained that due to the NP she was able to network and get experienced advice from progressives like Davis. Barack Obama, victor in the 13th State Senate District, encouraged NPers to join in his task forces on Voter Education and Voter Registration.

ME:  This has been a pet peeve of mine since I used to prosecute organized crime cases and had to deal with murderers and drug dealers who would argue that they weren't really "members" of a mafia family just because the fellas hadn't held that ceremony with the pricked fingers and burning saint cards.  Whether one is a "member" of some association is a function of our observation of his conduct; the niceties that the association in question imposes are relevant, but they're not dispositive.  For all it appears, Obama may very well have been a card-carrying member, but his degree of participation makes that question academic.