@Jameron did you ever find a solution to this? I'm currently trying to do the same thing.

Ah. Found a solution. The problem with passing in null is that there is a call like this

$this->createAttachRecords((array) $id, $attributes)

Which causes $id to become an empty array when you pass in null. To get around this you can pass in [null] which will stop the conversion and will then correctly add in a row where the id is null. Using your example

$group->members()->attach(
    [null],
    array(
        'first' => $new_array[$key][0],
        'last' => $new_array[$key][1],
        'favorite_food' => $new_array[$key][2]
    )
);