Despite you have this ability to create channels in types of Public or Private, you are able to send your public channel ID or its username to a private channel in order that other members join it easily as well. The only rule you have to follow is to use @ in your message.
In the Web version iGap you are able to have groups in private and/or public. To add more members to a public group needs the group address. It is effective to use the @ at the beginning of the group username and send it as an invitation link to a private group to invite members and make members of your public group more and more.