1) Create your group/forum or find an interesting website and then insert it as a web page in your google site.
Here is how to insert the page:
2) Click edit page > insert > more gadgets > search "Iframe Wrapper" > select > paste in the URL customise the display (uncheck display title, uncheck add border)
3) Click on ok and don't forget to save. And you are done! Check out the result!