Grower Central is hosted on Tronia's web server, so your customers will need to know how to get there from your company's web site.

You will be provided with the link destination for your company's Grower Central login page (it will be different for each retailer as we add your logo). You would then add a hyperlink with that destination URL to a page on your company's web site.

Your customers will go to your web site, click the link to the Grower Central login link, be redirected to our web server, use their Grower Central Username/Email and Password to log in to your agrē database, and view their account information.
(you can type the password, or let agrē generate a random one, and your growers can change it after they login)

I want to read the white paper (with lots of screenshots) about configuring Grower Central.