BuddyPress.org

I guess, you can use this approach

1st approach

1.Create a page template and put site wide activity loop there instead of the normal page stuff.

2.Go to Pages->new ,create a page and assign this template

2nd approach

1.Create a page template

2.Make this page template widgitized and call the widget in the page content area

3.Create necessary widget enabling login in your functions.php

4.Go create a page use this template.

6.Go to appearance->widgets and assign sitewide activity widget

It should work