If you are bringing a group, you can park your bus or van on the NORTH diagonal lane in front of the capitol. Tell the officer you are there for Human Services Day, and s/he will move the barrier to let you park.
You will need a sign for your front window, which you can make yourself as an expression of your creativity, or can be printed from this link (if it won't load in your browser, right-click the link, and download the MS Word document to print later).