This howto is written during alpha patch 2.6.3. The howto may or may not be relevant in later game patches.
There appears to be more to chat channels than this, but for now I’ll focus on switching between main/local and party chat, as lots of people seem to struggle with this. Local/main chat can only be seen by people on the same server as you, whereas party chat can only be seen by current members of your party, regardless of which server they’re on. Using local/main chat is the most effective way of making sure you’re actually on the same server as your friends.
Oh, and to toggle the chat box on and off, press F12.
When in a party, once you connect to a server, you’ll be in the party chat. You’ll get thrown back into party chat every time you join a new server as long as you’re in a party, or if you join a party after you’ve connected to a server. To select a different channel, bring up your cursor by pressing Right Alt (AltGr) + [the key next to your right shift key]. This would for instance be RAlt + / on a UK keyboard, and RAlt + – on a German keyboard.
This will bring up a few icons underneath your chat box. Press the second icon from the left, the one with the four horizontal lines on it. This will bring up the chat channels.
Simply press the channel you want to join.
Here I have switched to the main/local channel:
Normally main/local chat will be displayed in white, and party chat will be displayed in blue. You can also whisper to a certain user regardless of which server they are on, which will display in blue in whichever chat channel they are in. To whisper, use the command “/tell [user] [msg]”, for instance “/tell srbz hello, handsome” (without the quotation marks).