To delete a group you need to use the deleteGroup() method. The user must be an Admin of the group they are trying to delete. 
let GUID = "GUID";

CometChat.deleteGroup(GUID: GUID, onSuccess: { (response) in

  print("Group deleted successfully.")

}) { (error) in

  print("Group delete failed with error: " + error!.errorDescription);
}
The deleteGroup() method takes the following parameters:
ParameterDescription
GUIDThe GUID of the group you would like to delete