How do I add applicants to different groups?

How do we get applicants that come into RG to automatically be assigned to "Dog Applicant" or "Foster" or "Volunteer" depending on which form they complete?  Right now all of our applications are automatically going to "Contacts not in any group" category.  We have to manually mark volunteers and fosters and would like the groups to be marked automatically depending on which form they complete.  I don't see a section to category the form to a certain group within the creation of each form.