You can think of a channel group like a pointer to a list of channels on your server. If your application requires listening to large numbers of channels at once, channel groups allow you to send a single call which may subscribe to up to 2000 channels. Each individual client can subscribe to a maximum of 10 channel groups for a total of up to 20,000 channels.
You can't publish to a channel group. You can only subscribe to it. To publish within a channel group, you need to publish to each channel individually.
For more information, refer to Channel groups.