Compass - less than 300 active members
The results of the executive elections for soft left faction Compass are out here: http://clients.squareeye.com/uploads/compass/Compass1.xls
They reveal that it only has 274 members active enough to bother voting in its internal elections. You can - like former NUS President Gemma Tummelty - get on their executive with only 10 first preference votes.
Hardly a mass membership organisation poised to sweep to control of the Labour Party.