I bet this has been described before, but I did not find it with a quick forum search. So, here you go.
Per default, the chat tab for channels is only connected to "gossip". You can join any number of other channels:
/join blabla
This opens the channel named blabla. If you're the first to join, a new channel is created. Use number commands to send text to one channel.
/1 this text message goes to the gossip channel
/2 this text message goes to the blabla channel
Several channels in the same tab can be annoying. You can leave a channel any time:
/leave 1
This closes the channel bound to slot 1, gossip.
Then text you enter in this tab goes to blabla, and only to blabla.
Most players still prefer groups as ooc chat tabs. Some because they got used to it, some because they want to make sure only invited players take part in the chat.
/invite feas