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.
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!
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