Ever since Windows incorporated the taskbar and "automatic grouping", I've found it rather frustrating that there is no way to 'ungroup' a grouped taskbar button.
Example scenario: The taskbar is relatively full, and a new application is opened. This causes several instances of (say) IE to collapse into one button. This is automatic grouping. However, when that one new application is closed, the button does not automatically ungroup... even though there is obviously room to do so (since it was ungroups prior to the one application being opened and then closed).
In general, you have to close a lot more applications before the buttons will automatically ungroup.
Since there's a right-click menu on buttons, give some users some control. Add a "group" option to the right-click menu when there are multiple instances of an app open, and add an "un-group" option to all grouped taskbar buttons.
When "ungroup" is selected, if there is room, the buttons would simply be ungrouped. If there is not room, then either some other set of buttons would become grouped to make room, or if that wasn't possible (no other ungrouped multi-instances) then the task bar could go into its scrolling mode. If the user didn't like the result of the ungroup action, they could always right-click and group again.