Or in Elgg 1.7:

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

Obviously the Elgg 1.7 version is easier to understand.

Hey,

thanks a lot! I think I read over it without recognising that was the answer to my problem. Again I am positively surprised how fast you get a very precised answer in this community! Really thank you!

Greets,
Roberto