Im not shure if this is the best approach, but I would do the following.
Create a method in Page_Controller:
function MemberGroupText(){
$member = Member::currentUser();
$sqlQuery = new SQLQuery();
$sqlQuery->select = array('GroupID');
$sqlQuery->from = array("Group_Members");
$sqlQuery->where = array("MemberID =".$member->ID);
$rawSQL = $sqlQuery->sql();
$result = $sqlQuery->execute();
$groupid = $result->first();
return _t('MEMBERGROUPTEXT.'.$groupid['GroupID'], 'Default text');
}
create a language file in mysite/lang/ like en_US.php or nl_NL.php
$lang['en_US']['MEMBERGROUPTEXT']['2'] = 'Welcometext group 2';
$lang['en_US']['MEMBERGROUPTEXT']['3'] = 'Welcometext group 3';
In your template simply call the method when logged in with :
<% if CurrentMember %>
$MemberGroupText
<% end_if %>
In this way you always have a default text when a grouptext is not there and you can simply add textstrings for new groups in your language file..