Sorry, can't help you with that exactly, but the custom widget index that comes with the widget-manager plugin (highly recommended) has a special widget that lets you list group blogs in a "image-slider" like view. For me at least that's even better than "just" featuring groups.

The 'Featured Groups' lets the admin decide which groups to list...the 'Group News' allows each member to chose which groups (their Blogs) are listed.  Personally I think you would want them both available.

Thanks' Krischan and Clyde, your answers are appreciated but do not answer my question. Searching the Elgg site, I see that this topic comes up from time to time, but seems never to be completely resolved.

'Quick code fix' isn't in my abilities  @jon666 .  I have groups, featured groups and group news available for all my members to use, if they want, using the very plug ins you don't want to use. I don't have another method for you. 

Will try using code from groups/sidebar/featured.php and inserting it on custom_index.php. Don't see why that shouldn't work. The code is:

$featured_groups = elgg_get_entities_from_metadata(array(
    'metadata_name' => 'featured_group',
    'metadata_value' => 'yes',
    'types' => 'group',
    'limit' => 10,
));

if ($featured_groups) {

    elgg_push_context('widgets');
    $body = '';
    foreach ($featured_groups as $group) {
        $body .= elgg_view_entity($group, array('full_view' => false));
    }
    elgg_pop_context();

    echo elgg_view_module('aside', elgg_echo("groups:featured"), $body);
}

@jon666  Just do it

Wow, it actually works - and perfectly! Many thanks RvR for giving me that boost!