I figured it out, this is resolved
$past_args = array(
'paged' => $paged,
'posts_per_page' => -1,
'post_type' => 'exhibition',
'meta_query' => array (
'relation' => 'AND',
array (
'key' => 'ex_start',
'value' => $today,
'compare' => '<',
'type' => 'DATE'
),
array(
'key' => 'ex_groupsolo', // name of custom field
'value' => 'group',
)
),
'orderby' => 'meta_value_num',
'order' => 'DESC'
);