If you want only the active tab to show in the taskbar, then go to Groupy>Visuals>More Settings>"Only show a Taskbar button for the active window in a group."
If you want only the application to appear on the taskbar with all the tab thumbnails showing on Hover, then uncheck the above setting in Groupy and go to Windows Settings>Personalization>Taskbar>Combine Taskbar Buttons>Always, hide labels.
Play around with these settings and see if can accomplish what you want. 
