Online forms are added to the Group page as a separate step. It is optional to include an online form.
Group Admins
Go to : Group Detail > Forms > Add Form
View help article: How to Create an Online Form
School Admins
School Admins can add a form also by going to Online Forms > Forms Mgmt > Add Form. Create the online form, and in the last step set the audience for your group.
View help article: How to Create an Online Form