So apparently, it's illegal to "profess" that one belongs to a proscribed organisation and what's more, it has been since the 1970's. So just by saying, "I'm a member of the 17 November Revolutionary Organisation," I could get into heaps of trouble. Maybe if I removed the quote marks from that, I'd actually be professing rather than merely pretending to profess? Let's see...
I'm part of the Abu Sayyaf Group, and have a note to prove it. (Although according to the Home Office list, the "precise aims of Abu Sayyaf Group are unclear".)
On Tuesdays, I attend meetings of Al Gurabaa ("Al Gurabaa is a splinter group of Al-Muajiroon" - Al-Muajiroon don't seem to be on the list themselves though. Doesn't matter - I'm not a member of Al-Muajiroon .)
I did join Revolutionary Peoples' Liberation Party - Front (or Devrimci Halk Kurtulus Partisi - Cephesi) for a while, but being a spliter-of-a-splinter-of-a-splinter, they ran out of beer waaaay to soon.
Coming a bit closer to home, I was briefly a member of Cumann na mBan until my wig fell off and someone pointed out how deep my voice was.
All of which begins to sound like a challenge: how many proscribed organisations can one person belong to? Is there a record? Professing membership of ALL of them would be relatively easy (if illegal), given that there's only around 60-odd in total. But to actually BE a member of all 60 - to have a wallet with 60 membership cards, to know 60 secret handshakes, to have 60 lapel badges - that'd be way cool. Maybe a good stunt for Mark Thomas now that SOCPA's on the way out?
To be serious for a minute, proscribing groups is pretty serious. Although it can be as mild as "ostracism" according to the OED, in this context it's more like declaring them enemies of the state, saying these people have given up any right to exist. To arrest somebody for killing, shooting, bombing, gassing etc. civilians without an EXTREMELY good reason, is of course a Good Thing. If you kill because there's a voice in your head telling you to, that's sad & troubling & probably need psychiatric intervention - but you still need arresting. But if you call that voice "God" or justify the killing politically, then what's the difference? Murder is murder. Calling a murderer a terrorist is just glorifying terrorism, isn't it?
I was also a member of the Lego Builder's Club, c.1980-1982.
 Spelled the way the Home Office do, Google can only find Al-Muajiroon as a proscribed organisation. Wikipedia has Al-Muhajiroun as nasty but defunct. So panic over.