Enter a channel by clicking it's name on the tab. The "user list" in the upper-right corner of X-Chat contains each person's nickname in the current channel.

It's usually a good idea to just listen for awhile to get an idea of what's being discussed.

Compose a message by entering text in the "input box" next to your nickname near the bottom of the window, then hit "Enter" to submit it.

Any messages entered this way are read by everyone in the group. Most channels have more than only two participants as seen in these pages.