[TYPO3-mvc] Fetching FEUsers related to FEUserGroup

[Alex Bailey]

Hi all,
I have a problem which is driving me crazy.
At starters it doesn't sound like a big deal but I can't figure out how 
to get it to work:

I've mapped the feuser & feusergroup table to 2 extbase models I use called
Tx_ExtKey_Domain_Model_User
Tx_ExtKey_Domain_Model_UserGroup

this is working perfectly fine I can access both models in the frontend 
without problems.

The problem is the following situation:

When I try to fetch users which are in multiple groups ->
I have 1 User X which is in grp Fuu & Bar


When I query for all users in grp Fuu extbase doesn't find him.

I tried the following query methods:

$query->equals('usergroup',$usergroupObject);
$query->equals('usergroup.uid',$usergroupObject->getUid());
$query->contains('usergroup',$usergroupObject);
$query->contains('usergroup.uid',$usergroupObject->getUid());

But nothing seems to do the trick.
Sometimes I get the user but other people from the group are missing 
sometimes I only get 1 user and all the other ones are missing.

What is the right way to define a query to fetch all fe_users for a 
fe_usergroup?

I'm grateful for any answer,

regards

Alex