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.
For this purpose, you can tap on the three-line icon and choose “Contacts” among the list. Then you will see all the iGap contacts you have ever chatted with.
On top of the screen you see a frame named “Add Contact” in which you are able to enter the new contact’s info in order to add.
When you scroll down the List of Phone Contacts is visible who may not join iGap yet. So, if you want to ask them for joining just send them an invitation.