I haven't worked with classes all that much, but I do see a couple of things.
1. You've created a variable in your class module, TextBoxGroup but it doesn't point to anything.
You need to add:
Public Property Set TextBoxGroup(txt as Textbox)
Set TextBoxGroup = txt
End Sub
to your class.
2. Access has an optimization built-in that if it doesn't see '[Event Procedure]' for an event property, it doesn't raise an event. So you need to set that. You can do this in the class Set:
Public Property Set TextBoxGroup(txt as Textbox)
Set TextBoxGroup = txt
TextBoxGroup.OnDblClick = "[Event Procedure]"
End Sub
3. Last, as Helen has said, I don't think you can use an array like that. instead, you need to have your class handle a single text box and then use a collection in the form to build an array of controls. I'll see if I can't dig up some code or someone who has worked with classes a lot more.
JimD.