SharePoint offers a web part "Send mail to all members" . This when clicked should normally draft/send a mail to all the members of the SharePoint site.
However this works only if there are very few members on the SharePoint site.
The feature/Web part may not work when there are too many users on the site. There is a length limitation in the URL link formed by the Mailto href tag. When the length exceeds 2048, the mailto link will not work. Please see this KB articles for detailed info.
http://support.microsoft.com/kb/208427/en-us However there is a workaround for this issue as recommended by Microsofthttp://support.microsoft.com/kb/182985/en-us