When You Create The Form
When you create a Smart Form, you'll select your beneficiary and petitioner (if relevant). At that time, if you would like to invite either contact to the Smart Form, just check the box right under the contact's name. The contact will immediately receive an email inviting them to complete the form.
After Creating The Form
If you want to invite a contact after creating the form, that's fine too! In the forms tab in your dashboard, click the link under the Invites column next to your form. From there, you'll be able to invite contacts as well as monitor whether your invitation has been sent, accepted, or returned for review. Here's a screencast you can follow along.