https://piazza.comand log in
Click the drop down menu in the upper left corner of the screen where you select your classes (Labeled #1 in the picture).
Below the list of all your classes, select the link that says "Add Another Class" (Labeled #2 in the picture).
Click the Selected Term Dropdown Menu, and select the term called "Other".
Type in "GreyHat" into any of the class input (Labeled #1 in the picture) After Searching, GreyHat should appear in the menu beneath. Click the first option that appears.
If it appears, select "Join as Student" in the dialog option that appears. Then, hit the "Join Class" button (Labeled #2 in the picture).