The templates that I wish to group start with NewTemplategroup. And this is what I found:
Use newtemplategroup in lower-case as the array index, otherwise it doesn't work
// This works like a charm

$only['newtemplategroup'] = 'New Template group';
// This does not work

$only['NewTemplategroup'] = 'New Template group';
Thanks for the quick reply. I hope this helps someone