Only members of a group channel can invite other users into the channel. You can also determine whether or not to show the past messages prior to the user's joining of the channel.
Note: In your dashboard, go to the Settings > Chat > Channels > Group channels, and find the Chat history option. If this option is turned on, new users can view all messages sent in the channel prior to their joining. If turned off, new users only sees messages sent after they've been invited. By default, this option is turned on.