How to allow sellers to register on my store and configure thier items which they sell?

Currently in demo site I cannot see any option where sellers can register themselves like other ecommerce store has e.g. Amazon, Flip-kart, etc.
Can anybody guide me?
Pardon for my English.

In nopCommerce sellers call vendor. You need to turn on such a possibility (Configuration >> Settings >> Vendor settings).
If you are testing on demo.nopCommerce.com, you should know, that front end and back end are not connected. So any changes made on admin panel will no effect on front end. For test purpose I would recommend to download software and install it on your local machine, or server if you have one.

If you do not thinking about vendors, rather some kind of announcement board, then I would suggest to use plugin.

As here Nop4You has pointed out in a right way!

You can find a similar post here:

Is there a vendor login? Where can I see its demo?

And
Vendors Subscription or Registration - Invite a vendor - New vendor request

And also issue on CodePlex:

New vendor request