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Strictly speaking, wp-load.php is the handlet for all AJAX calls, in virtue of the WP AJAX API.

The key to finding the function that processes data is to look in bp-themes/_inc/ajax.php for an add_action hooked to wp_ajax_[youraction]. For instance, since in this case the action being passed is members_filter, search for wp_ajax_members_filter, and the function hooked to it.