We do this intentionally, only group creator can appoint admins, for security reasons. It can be easily changed by very small modification, but we strictly don't recommend any modification and on UNA Cloud sites it isn't allowed.

In modules/base/groups/classes/BxBaseModGroupsModule.php file, the following code:

    public function checkAllowedManageAdmins ($mixedDataEntry, $isPerformAction = false)
    {
        if (is_array($mixedDataEntry)) {
            $aDataEntry = $mixedDataEntry;
        }
        else {
            $oGroupProfile = BxDolProfile::getInstance((int)$mixedDataEntry);
            $aDataEntry = $oGroupProfile && $this->getName() == $oGroupProfile->getModule() ? $this->_oDb->getContentInfoById($oGroupProfile->getContentId()) : array();
        }


        return parent::checkAllowedEdit ($aDataEntry, $isPerformAction);
    }

need to be modified to:

    public function checkAllowedManageAdmins ($mixedDataEntry, $isPerformAction = false)
    {
        if (is_array($mixedDataEntry)) {
            $aDataEntry = $mixedDataEntry;
        }
        else {
            $oGroupProfile = BxDolProfile::getInstance((int)$mixedDataEntry);
            $aDataEntry = $oGroupProfile && $this->getName() == $oGroupProfile->getModule() ? $this->_oDb->getContentInfoById($oGroupProfile->getContentId()) : array();
        }


        return self::checkAllowedEdit ($aDataEntry, $isPerformAction);
    }