I think the doc on the module page is quite good, but to try and help...
take - http://doc.silverstripe.org/datamodel#many_many
in both dataobject add some static $db (including 'Name')
in Team add...
function getCMSFields()
{
$fields = parent::getCMSFields();
$fields->removeFieldFromTab('Root','Categories');
$dos = DataObject::get('Category');
$map = $dos ? $map = $dos->toDropdownMap('ID','Name') : array();
$fields->insertBefore(new MultiSelectField('Categories','Categories, $map),'Name');
}
then in ModelAdmin you should see a nice multi select field... not the above is untested... but it should work... let me know and I'll edit it up for a good example.