I just used the following code:
<form method="post">
<input id="user1" value="user1" name="invite[]" type="checkbox">
<input id="user2" value="user2" name="invite[]" type="checkbox">
<input type="submit">
</form>
<?php
if(isset($_POST['invite'])){
$invite = $_POST['invite'];
print_r($invite);
}
?>
When I checked both boxes, the output was:
Array ( [0] => user1 [1] => user2 )
I know this doesn't directly answer your question, but it gives you a working example to reference and hopefully helps you solve the problem.