Side chats are separate conversations with people in the conference, where you can plan ahead for the conference or talk about private or sensitive subjects.
You can add side chats by clicking on the plus icon on the bottom of the screen. Adding people in the side chat is done the same way as with conferences, where the list of contacts is changed to the list of people in the conference.
All of the side chats you are in are listed next to the plus sign. You can easily go in by simply selecting the side chat and then go out by selecting another side chat or the main conference icon.