ACF stores a list of user IDs as a serialized array.
The meta key of the field will be "{$group_field_name}_{$sub_field_name}"
your meta query might look something like this
'meta_query' => array(
array(
'key' => "{$group_field_name}_{$sub_field_name}",
'value => '"'.$user_id.'"',
'compare' => 'LIKE'
}
)