I searched around I cannot find the sapphire's forum, wiki, developer's group etc.
So Silver Stripe is for people who edit the web content. Sapphire is for people who build web sites.
My questions are
1) As a web developer (php programmer) not a web graphic designer, what will be his works in developing web sites by using SilverStripe? I already installed the SilverStripe, and quickly try it, it seems to me silverstripe is for web designer who doesn't have to know php. I really want to know that if a web project on silverstripe requires a php web developer to work with web graphic designers, what kind works php web developers are needed in using SilverStripe?
2) Where are the developer's guide, forum, wiki, developer's group etc just for sapphire?
Sapphire is a PHP framework and SilverStripe is a CMS that's based on said framework.
To effectively bring the CMS to a good use, PHP knowledge is mandatory in my opinion (only to build the site, not to administrate it of course). Sure, you could copy-paste some snippets together, but to get the best results and highest flexibility you'll write custom classes for your website needs.
Since the cms and the sapphire framework are built with an MVC structure in mind, you can easily split the designer and programmer part. Designer might create CSS, Images and Templates, while the programmer writes all the required functionality (classes, controllers etc.)
A lot of the documentation you'll find actually covers sapphire, since SilverStripe is really "just" the CMS module on top of it, and it obviously makes heavy use of the framework classes. It's probably best you have a look at the classes and their relationships. Here are the API docs: http://api.silverstripe.com/ (select "sapphire" on the top right to see the sapphire classes).
Or probably even better, start with the tutorials listed here: http://doc.silverstripe.com/doku.php
You'll find out pretty soon about the inner workings of the cms/sapphire.
I searched around I cannot find the sapphire's forum, wiki, developer's group etc...
Well sapphire is made up of lots of components and a few of those have their own forum - namely [url=http://silverstripe.org/data-model-questions/]Datamodel[/url], [url=http://silverstripe.org/template-questions/]templates[/url] and [url=http://silverstripe.org/form-questions/]forms[/url] are all Sapphire based rather then CMS focused.
The CMS and Sapphire are tightly integrated so most of time the documentation refers to features in sapphire.