$db = JFactory::getDbo();
$db->setQuery("SELECT `memberid` FROM `lc_community_groups_members` WHERE `groupid` = 1 AND `approved` = 1 ");
$column = $db->loadColumn();
return(implode(',',$column));

$query->select($db->quoteName('b.email'))
        ->from($db->quoteName('#__community_groups_members', 'a'))
        ->leftJoin(
            $db->quoteName('#__users', 'b')
            . ' ON (' . $db->quoteName('b.id')
            . ' = '
            . $db->quoteName('a.memberid') . ')'
        )
        ->where($db->quoteName('a.groupid') . ' = 1 ')
        ->where($db->quoteName('a.approved') . ' = 1');

$db = JFactory::getDbo();
$query = $db->getQuery(true);

$query->select($db->quoteName(array('a.*', 'b.memberid')))
      ->from($db->quoteName('#__community_groups_members', 'a'))
      ->where($db->quoteName('groupid') . ' = 1 '. 'AND')
      ->where($db->quoteName('approved') . ' = 1')
      ->join('INNER', $db->quoteName('#__users', 'b') . ' ON (' . $db->quoteName('b.email') . ' = ' . $db->quoteName('b.id') . ')')
      ->where($db->quoteName('b.id') . ' = ' . $db->quote('b.memberid'));

$db->setQuery($query);
$column = $db->loadColumn();

$db = JFactory::getDBO();
try {
    $query = $db->getQuery(true)
                ->select("GROUP_CONCAT(DISTINCT B.email ORDER BY B.email)")
                ->from("lc_community_groups_members A")
                ->innerJoin("lc_users B ON A.memberid = B.id")
                ->where("A.groupid = 1 AND A.approved = 1");
    //echo $query->dump();
    $db->setQuery($query);
    if (!$emails = $db->loadResult()) {
        echo "No Qualifying Email Addresses";
    } else {
        echo $emails;
    }
} catch (Exception $e) {
    echo "Syntax Error"; // . $e->getMessage();
}

|  lc_community_groups_members        |  JOIN ON memberid=id  |         lc_users        |
|-------------------------------------|                       |-------------------------|
|  memberid  |  groupid  |  approved  |                       |  id  |       email      |
|------------|-----------|------------|                       |-------------------------|
|      1     |     2     |      1     |           <           |   1  |   a@email.com    |
|      2     |     1     |      1     |           <           |   2  | shared@email.com |
|      3     |     1     |      0     |           <           |   3  |   c@email.com    |
|      4     |     1     |      1     |           <           |   4  | shared@email.com |
|      5     |     1     |      1     |           <           |   5  |   e@email.com    |
|      6     |     2     |      0     |           <           |   6  |   f@email.com    |
|      7     |     1     |      0     |           <           |   7  |   g@email.com    |
|      8     |     1     |      1     |           <           |   8  |   h@email.com    |
|      9     |     1     |      1     |           <           |   9  |   i@email.com    |
---------------------------------------                       ---------------------------

e@email.com,h@email.com,i@email.com,shared@email.com