You are being asked to approve or deny group membership requests because one or more of your groups is set to “Members must be approved by admin” in the Joinability section of the group settings.
To view or edit this setting follow these quick steps:
1. Select “Manage” from the Groups section of the dashboard
2. Select the desired group from the Groups page and click “Actions” (found to the far right of the selected group)
3. Click “Edit”
You will find the “Joinability” section in the lower right hand corner of the screen and update your preferences there.