This input is different from the others because it doesn't add an input type to the UI.MultipleInputForm++ forms. It was only designed to let you visually group inputs on the form in order to give your users a better understanding of the different "sections" of inputs.
GroupTitle (optional): This input allows you to add a title to your group of inputs.
Inputs: This is where you need to feed all the inputs you wish to group together on the form. The inputs must be joined in a list (using List.Create).
The InputGroup input type doesn't return any output of its own. It does'nt have any concequence on the way inputs are returned by UI.MutipleInputForm++.
The node will only take a list of items to group so if you are trying to group a single input you need to add a List.Create after the input you are trying to group and the UIInputGroup Data