How to scedule a reminder mail for nodes with date field in an oganic group?
When it comes to right on schedule or specific time, you need to have crontab on server. Role of cron tab will be none other than calling your cron.php file which in turn allows all modules to set the cron. But this is half of the process since with core drupal cron process you can not have much control on setting schedule time, so you can use elysia cron module to extend drupal core cron.This was cron part

Now inorder to trigger email you can implement hook_cronapi() as an example,

function mymodule_cron_cronapi($op, $job = NULL) {  

  $items['send_email'] = array(
    'description' =&gt; 'Send email to group users',
    'rule' =&gt; '0 * * * *', // Every Hour at **:00
    'callback' =&gt; 'mymodule_email_group_callback',
  );
  return $items;
}


cron callback

function mymodule_email_group_callback() {
  // load all group content from that content type.
  // loop through all nodes check date
  // if true
  // get all subscribers and loop through them
  // send email
}