I'm not sure what you mean by add them to the page? Also I have never heard of shapes and beats but sounds fun. The idea for this actually stemmed from Undertale.
You can add contributors to a project by going into the edit page of the product.in the header there should be a "More" dropdown, and within that there is an "Admin" section.within the Admin section you can add people to the project using their itch username or profile url.
oh lol they don't have itch accounts