You can see the list of contacts whom you have already chatted with in iGap, either add more members or invite other non-users among the contacts list of your phone.
In iGap through Web version, there is this feature to create new channels. Channels are as Public or Private. In private one you decide who to be added to your channel but in public every one of iGap users can join it and enjoy the shared media. So, try to make your decision about your channel type first and then make use of it.
You can create new groups in iGap using the option New Group and control and administrate it easily. In both Web and Android version, you are able to have either private or public ones. It's possible to have a private group with your friends known to each other or a public group in order to make new friends and get acquainted with more people.