I can not add any type of control to the layoutControlGroupItem. I have a screen recording of me trying to follow the procedure shown but it is 60M so I can't attach it. I shared the file on my OneDrive if you would like to access it that way.
radlayoutcontrol does not support System.Windows.Forms.TextBox autosize in multiline mode but supports radTextBox, Please considere fizing that bug. take a look at the attached picture