This is not supported by the layout control tab group. I logged a feature request in our feedback portal where you can track its status and vote for an implementation. I also updated your Telerik points.
You can achieve your requirement by extracting the ControlTemplate of the LayoutControlTabGroup element and wrapping the ItemsPresenter that shows the tabs into a ScrollViewer. You can see this approach demonstrated in the attached project.
Telerik by Progress
Telerik UI for WPF is ready for Visual Studio 2017 RC! Learn more.