array (
    0 => array (
        'invoice_id' => 'WUI_588'
        'email' => 'abc@hotmail.com'
    ),
    1 => array (
        'invoice_id' => 'WUI_588'
        'email' => 'def@hotmail.com'
    ),
    2 => array (
        'invoice_id' => 'WUI_589'
        'email' => 'ninja@hotmail.com'
    ),
    3 => array (
        'invoice_id' => 'WUI_589'
        'email' => 'ghi@hotmail.com'
    ),
    4 => array (
        'invoice_id' => 'WUI_590'
        'email' => '123@hotmail.com'
    ),
    5 => array (
        'invoice_id' => 'WUI_590'
        'email' => '123@hotmail.com'
    ),
    6 => array (
        'invoice_id' => 'WUI_591'
        'email' => '456@hotmail.com'
    ),
    7 => array (
        'invoice_id' => 'WUI_591'
        'email' => '456@hotmail.com'
    ),
)

INDEX | INVOICE ID | EMAIL
  0   |   WUI_588  | abc@hotmail.com
  1   |   WUI_588  | def@hotmail.com
  2   |   WUI_589  | ninja@hotmail.com
  3   |   WUI_589  | ghi@hotmail.com

INDEX | INVOICE ID | EMAIL
  0   |   WUI_588  | abc@hotmail.com, def@hotmail.com
  1   |   WUI_589  | ninja@hotmail.com, ghi@hotmail.com

foreach ($array as $key => $value) {
  $invoiceidbucket[] = $value['invoice_id'];
  if(in_array($value['invoice_id'], $invoiceidbucket)) {
    $value['merged_email'][] = $value['email'];

    //Clearing array
    $invoiceidbucket = array;
  }
}

$final_array = array();
foreach($initial_array as $arr){
  $final_array[$arr['invoice_id']]['invoice_id'] = $arr['invoice_id'];
  $final_array[$arr['invoice_id']]['email'] = (!empty($final_array[$arr['invoice_id']]['email']))?$final_array[$arr['invoice_id']]['email'].','.$arr['email'] : $arr['email'];
}
$final_array = array_values($final_array);
echo "<pre/>";print_r($final_array);

$a = [];//Your array;
$retArray = []; //Output Array
foreach ($a as $key => $val) {
    $retArray['invoice_id'][] =  $retArray['email'];
}

$array=[
    ['invoice_id'=>'WUI_588','email'=>'abc@hotmail.com'],
    ['invoice_id'=>'WUI_588','email'=>'def@hotmail.com'],
    ['invoice_id'=>'WUI_589','email'=>'ninja@hotmail.com'],
    ['invoice_id'=>'WUI_589','email'=>'ghi@hotmail.com'],
    ['invoice_id'=>'WUI_590','email'=>'123@hotmail.com'],
    ['invoice_id'=>'WUI_590','email'=>'123@hotmail.com'],
    ['invoice_id'=>'WUI_591','email'=>'456@hotmail.com'],
    ['invoice_id'=>'WUI_591','email'=>'456@hotmail.com']
];
foreach($array as $row){  // iterate all rows
    if(!isset($result[$row['invoice_id']])){  // if first occurrence of invoice_id...
        $result[$row['invoice_id']]=$row;     // save the full row with invoice_id as the temporary key
    }else{                                    // if not the first occurrence of invoice_id...
        $result[$row['invoice_id']]['email'].=", {$row['email']}";  // concatenate the email value
    }
}
var_export(array_values($result));

array (
  0 =>
  array (
    'invoice_id' => 'WUI_588',
    'email' => 'abc@hotmail.com, def@hotmail.com',
  ),
  1 =>
  array (
    'invoice_id' => 'WUI_589',
    'email' => 'ninja@hotmail.com, ghi@hotmail.com',
  ),
  2 =>
  array (
    'invoice_id' => 'WUI_590',
    'email' => '123@hotmail.com, 123@hotmail.com',
  ),
  3 =>
  array (
    'invoice_id' => 'WUI_591',
    'email' => '456@hotmail.com, 456@hotmail.com',
  ),
)