Allow Anonymous user to Subscribe while site under maintenance mode The title says it all.
I have customized the Maintenance mode page, to Coming soon!!
how i want to enable the visitors of the website to Subscribe So they'll will be notified when the website is officially lunched.
how can i Achieve this, am developing the website in Drupal in 7.
Maintenance mode is not really for this.

I would recommend using a custom theme for "Coming soon".   That way you can have them come to a home page with information and sign up/register for the site.

When you're ready to put the real site up, change themes.