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;
?>