Yes, it was careless of me to say "joined the party." A quick shufty around found this (Rowena Mason in 2019 in the Guardian):

"Figures from the party’s latest accounts show it had 518,659 members in December 2018, down from a peak of 564,443 the previous year."

I heard 600,000 somewhere but I can't find it, and I'm not inclined to spend more time on it.