This event has long passed...
The Member Card is a key event item available in Pokémon Platinum. It is only obtainable through a Nintendo Event.
Aug. 3 to Sept. 13, 2009
Source: Bulbapedia/ Member Card
Not only has the event passed, but it is a Key Item, so no one can give it to a Pokemon to hold a trade to you for you to use. But maybe there will be a future event. Who Knows