You can select a role that will be moderators for all the groups on your site. These moderators will be able to approve or deny all groups, so select a trustworthy role. We suggest that you choose the Administrators Role, but any custom role can be used also.

The enable this option, go to the Global Group Settings in the control panel, groups tab. Here you will see a drop down list with all the roles on your site. Select one and click Add Role. You will notice the role shows up in a little box.  Currently you cannot delete the role once you add it, so choose carefully. You can add multiple roles if you need to, by following the same steps.