I am thinking in Ireland of the Socialist Party and the Socialist Workers Party.
There are a couple of linked things to know about these parties.
They are Trotskyite. Their figure-head symbolic leader is Leon Trotsky of the Russian Revolution. So they are Communist really. I think a lot of people know that they are Communist but not specifically Trotskyite, whoever cares. Personally I canvassed for Joe Higgins and the SP for the 2004 European and Local Elections and was in touch with them monthly for a year later without knowing they are Trotskyite. They told me they weren't Marxist or one of them did so I presumed they weren't Communist at the time. I decided to canvass for Higgins after having a few beers at home on my own. So I was a bit drunk when I emailed them saying that I would work for them for the then 04 EE and LE. The demon drink! I am sure that I wouldn't have made the decision if I was sober but I didn't feel strongly either way so the next day I just went with my decision.
The SWP are middle class I think. Middle class students a lot of them are I have heard. The SP Youth Wing Socialist Youth would be mostly full with (middle class) students as well but their general members are working class.
Personally I found a lot of SP members to be scarred psychologically. They come generally from poor backgrounds and I know at least one of their councillors and one of their old candidates come from very large, unusually large actually, families. So they have a lot of problems and a lot of anger or hatred. Joe Higgins can be funny of course but personally I found him to be often nihilistic personally.
So my advice, on my own experience, would not be to have anything to do with the SP and I suspect SWP also. If you are interested make sure you agree with what Leon Trotsky stood for first. I was in college with a fella in the Communist Party. In Ireland they are probably, from my very limited experience, the most harmless communist party there is funnily enough.
I think to join a far-left party is a sign of political immaturity, naiviety or well just innocence really. I never joined the SP but I suppose I just might as well have had. Eckhart Tolle says that our entire world has schizophrenia in the popular sense of the word (if you get me) and Mark Durkan once said that the DUP have or had "political schizophrenia". I think this applies to far-left parties and it took me a few years, although I didn't think about them every day of course, to put clear blue water between me and them and finally get away from their psychotic outlook. I never bought into their psychotic outlook- which I believe what they stand for i.e. Trotskyism to be- but I was effected or rather affected by it for a while. Time heals all wounds though. It took me around five or six years. Now the SP have a branch in my area and Joe Higgins got the most votes in my estate in the '09 locals and Euro's last year. I just ignore them if I see them. I don't say hello. They are going nowhere and I am away from that psychotic viewpoint now thank God.
Sorry for rambling but I think that it true that dozens of innocent, if only one or two, young people (generally) join far-left parties every year and then get scarred by their experience and often unfortunately let themselves become disillusioned and turned off from political activism for a long time or life which I believe to be a sin (well you get me). As I said though time heals all wounds. I believe that my stint with the SP played a big part in making me stronger politically. Do any of you have any experience of these parties?