site offline but allowing some visitors to login

Hi!
I created a wordpress site a few months ago using Vantage Premium theme, wich is great. I’m not very skilled but your tutorials and forum were very hellpful. The site is almost done but it’s still offline (I use the “Site Ofline or Coming Soon” plugin).
While the site’s not finished, I’d like to keep it offline but I’d would also want to allow some specific visitors to see it. Could you tell me how can I do that?
Thank you very much.
Greetings
Gon