The Liberal democratic party D66 added the largest number of new members last year, according to figures from the political party documentation centre, quoted in the NRC.

The party welcomed 6,200 members in 2009, taking its total membership to 18,500. D66 is still one of the smallest parties in the country but is currently riding high in the opinion polls under the leadership of Alexander Pechtold.
The Socialist Party SP appears to have been hit by the loss of its charismatic leader Jan Marijnissen. The party lost around 4,000 members, or 8% of the total, the NRC said.
The Labour party (PvdA) lost almost 2,000 members, or 3.4% of the total.
The NRC article does not list the other party membership movements. Geert Wilders’ anti-Islam party PVV and Rita Verdonk’s ToN do not have any official members and are not included in the statistics.