https://elgg.org/search?q=adsense&search_type=all
Maybe this plugin is of some help: https://elgg.org/plugins/2208299.
I don't use ads so I can't give you any specific advice about how to use adsense. Maybe the Adsense plugin works for you. Or it might give you at least some starting point for implementing the adsense usage on your site. For creating the code to ad adsense support you most likely would only need some knowledge about extending views (http://learn.elgg.org/en/stable/guides/views.html#altering-views-via-plugins) to add the adsense code whereever you want the ads to appear on your site.
@nishant kumar You can use this Google AdSense Elgg plugin
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