On Android version of iGap messenger, when you choose “new group” in the menu and add your desired contacts, the group will be a private one.
You need to click on the bar above and open the group settings to make it public.
On the top right corner, you see a three-dot icon. Click on the icon. Two options will be shown. By choosing the second one you can turn your private group into a public one. The notification of public group asking if you are sure to make the group public will be displayed to you.
By confirming a username will be activated for you. You can put your username from 5 to 32 characters. After the final confirm, select the save option to finalize the process and make the group public.