Use Login as user to log in as a specific user within the current domain.                        

1. Click Login as user in the Speedbuttons under Tools.

The Login as user window opens with an overview of all users within the current domain.                                                              

2. Select which user you want to log in as in the overview.o 3. Click Login in the lower right corner of the window                       

You will be logged in as the selected user. This can be seen in the top left corner of the screen. 

4. Click Sign Out in the top right corner of the screen to sign out as the selected user.