Due to the restraints placed on adding custom HTML code to Google Sites, you are presently unable to add a Promotion Box for your Google Group(s).  A possible workaround for this issue is outlined below.

1. Log into your Site.
2. Navigate to the page you wish to include the Promotion link on.
3. Click the "Edit Page" button.
4. Select the text that you would like to use for your link (or type it in if it does not yet exist).
5. Click the "Link" button on the edit toolbar.
6. Select the "Web Address" tab.
7. Enter a link to the signup page for your Google Group:
   
   http://groups.google.com/group/GROUPNAME/subscribe
   

   (GROUPNAME = The name of your group.)

This will create a basic link that will get your users to the subscription page for your Google Group(s).  It's obviously not seamless as the promotion box would be, but until Google makes it a little easier, this is an optional workaround.

Have a question, but don't know where to start?  Check out the Google Sites FAQ!