If this option is disabled or greyed out on your Windows 7 system, there are a couple of things you can do to fix it. You have to edit the registry to get it to work.
Method 1 – Registry
You can re-enable the Switch User option by navigating to the following registry key:
You should see a value called HideFastUserSwitching with a value of 1. A value of 1 means that user switching is disabled. To enable it, change that value to 0. Note that if you don’t have the HideFastUserSwitching key at all, you can create one by right-clicking in the empty space on the right pane and choose New – DWORD (32-bit) Value.
Set it to 0 and then exit the registry. You will have to log off and then log back on for the changes to take effect. When you click on the arrow next to shutdown now, it should be enabled.
Method 2 – Download Switch User Tool
A small little tool that simply switches users whenever it is run.