// for set to false of no value
$args = array(
'numberposts' => -1,
'post_type' => 'team',
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'is_manager',
'value' => 0,
'compare' => '='
),
array(
'key' => 'is_manager',
'compare' => 'NOT EXISTS'
)
)
);