If you're looking for fansubbing channels with a lot of people in it, you could try looking through /list. The command will give you a list of non-secret channels on the network in decreasing order of population.

And like you said, it may be a good idea to go through your folder to check whether the group added their IRC channel on their releases. I'm a fansubber myself, and if you know the group's name but you're having difficulty finding their IRC channel, you can ask me on here or via IRC. I'll see if I can help you (but I can't guarantee anything!).