I did a "help channel" and did not see a command for joining a channel.

where are there docs on the client?

You were one letter off, 'help channels' ingame for some info. In short, you don't need to "join" a channel. There are 2 ways to turn it on. You can type 'newbie on', or you can just talk on the channel, and if it was off, it'll get turned on.

--ooga

And just type channels to see what channels are available.