Run a check for multiple meta key values

use php’s implode to join array elements with a string:

<?php
if( $bands = get_post_meta($post->ID, 'band') ):
    ?>
    <strong>Band:</strong> <?php echo implode( $bands, ', ' ); ?><br />
    <?php
endif;
?>

EDIT- another version of above, pluralizing the label depending on single or multiple meta values:

<?php
if( $bands = get_post_meta($post->ID, 'band') ):
    $label = count( $bands ) > 1 ? 'Bands' : 'Band';
    ?>
    <strong><?php echo $label; ?>:</strong> <?php echo implode( $bands, ', ' ); ?><br />
    <?php
endif;
?>