One of the most commonly asked about symbols in Scheme-it is the ground symbol or as it is referred to in Scheme-it the ground Port. If you click on the Schematic Symbols on the left side of the screen first.
You should then see ports pop up as an options. These are in alphabetical order. When you click on ports you should see the options pop up on the bottom of the screen. In my window I have 29 options. You can then select the one you want to use and drag it into your schematic.
There are a couple other options that are popular that can be found here.
Here is a popular port in scheme-it. Some of our customer also refer to this an an endpoint. I have also seen it used as a test point.
You can see in this diagram it is used in several locations for input and output.