Try this
$query = "SELECT GroupName, Groups.idGroup
FROM Groups
INNER JOIN Members
ON Groups.idGroup = Members.idGroup
WHERE Members.idMember = ? ";
//prepare query and execute
if($stmt = $this->connection->prepare($query))
{
$stmt->bind_param('s', $this->id);
$stmt->execute();
$stmt->bind_result($GroupName, $idGroup);
$helperArray =array();
while ($stmt->fetch())
{
$helperArray[] = $idGroup;
}
print_r($helperArray);
}