How do I allow members of my multisite to just edit the main site?

Hi,

How do I allow wordpress multisite admins to update the main site. I want site subscribers to add pictures of products they want to sell and post them with a price on our product page. I do not need them to create a seperate site to do this.

How can I help them avoid making a seperate site to sell their products.

The following is working:
Wordpress Multisite
Webriti SMTP Mail (they recieve activation email)

The following is not working:
1. Member dashboard, because they click on menu items and cannot enterpage. Error
2. Users cannot enter through login link. (Error Attatched)

What do I have to do to allow users to just update the main site with pictures and products using the products page?

Thanks