If you have enabled Offline Payment for the Group, you will be able to locate the details of those requests by going to the Group Dashboard > Members > Pending Payment.
In the Pending Payment section, you can view the list of people who have selected Offline Payment while buying the membership plan. You can either download these records or you can see a quick view by clicking on "i" button next to the particular line.
Once you confirm that payment has been received, you can come here and click on the "Manage" icon and click on "Confirm". This will immediately send a confirmation email to the person informing them that the membership has been confirmed.