Q:

Enable certain users to add products to my store

I'd like to allow some users to sell products in my store, without giving them access to magento's back end. 
Is there any extension for doing that ?

A:

This functionality is commonly known as an online marketplace. There are a number of extensions that do this, the most popular of which by far is Unirgy Dropship. It is an expensive, commercial, solution. 
This has been covered a few times here before:
How to setup multiple (isolated) Magento admin views (multi-vendor)
Etsy style multi vendor site

