You can add Contacts to Playbooks in 2 ways:
Option #1) Bulk Enrollment:
Select multiple Deals using the checkboxes
Click the ‘Playbooks’ Button
Click "Add to Playbook"
Select the Playbook you want to use
The contact for each deal will be enrolled
When bulk enrolling Deals, the Deal's contact will be engaged. In most cases, this is the Primary (top most) contact.
Option #2) Manual Enrollment:
Open a Deal
Locate a Contact
Click the Playbook icon
Click "Enroll" next to the Playbook you want to use