What if the new member forgets to indicate how did he heard bout the group? How can the person who referred him gets his monthly commissions? can the admin update the info? If its possible how will I do this.?

If you referred an account to us but they didn't go through your affiliate link (Or otherwise indicate they were referred by you), you can just let us know, and we'll check with the customer, and if they confirm, we'll add your affiliate ID to their account.

Actually its within our igroop. A member referred a client and the client did not indicate how did you hear about this group?

As admin of the group how can I solve this problem? Is it possible that I be the one to update it?

View the profile of the new member. There's a field called "Affiliate ID". Enter the nickname of the referring member. This will give the referring member credit for inviting the new member.