Navigate to the GROUPS page:
Click on Pending Groups in the left-hand column of the page:
Then you will see basic information on the list of groups awaiting approval, and simply click on Approve or Deny on the right.
NOTE: The setting to require network admins to approve or deny groups is located in the General section of your Network Settings page:
When a member tries to create a group, and e-mail is sent to administrators with a link to the approval page, and after approval or denial is given, the requester receives an email saying their group is ready, or that they got denied.