This article will explain what the posting group do for the debtors and creditors.
The posting group is a way to assemble all of your debtors and creditors in the chart of accounts. This is being set under Settings -> Finance -> AR/AP groups.
Here you can also set the reporting group for your debtors and creditor.
After you have chosen which account the posting group should be in then you can chose the correct posting group for your debtors or creditors.