To make it so that only the group administration can invite new members:
  1. click "Settings" under the main group picture;
  2. select "Privacy settings" in the menu on the left;
  3. set "Invite new members" to "No" in the "Allow all users to" section;
  4. click "Save" to confirm.
Note that this setting is only available for closed groups and can only be accessed by group administrators.
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