For me, you join them 10 years ago and never leave. The same people are still there, maybe some new ones joining every now and then. People find them out by other people asking them to join. My favorite entertainment is definitely this one Finnish music channel, where the chat can be hilarious at any given time.

In these channels there's people who've been there for almost 20 years. Sadly I parted ages ago from the channels I was chatting in the 90's. But I guess they're still there.

Join a ton of channels. Lurk for a bit, add things into the conversation. After a month or so everyone will be comfortable with you and you just keep going. More general topic channels like #startups on freenode are a bit easier to be social in. Learn the local rules and the local users. It's like any community really.

I've found that $"{someOpenSourceProject}-offtopic" sometimes works.

Other than that, I just see people mention channels sometimes and I join them.

http://irc.netsplit.de/ is a good resource. Read up on the network before joining a channel. Each has their own culture, which is somewhat important to know before you lurk around.