When editing a single bulk email or drip campaign, you can manually add contacts to the email list.
Click the "Email List" section and click the "Add Recipients" button in the right corner. You have 3 different ways to add contacts to the list:
Search Contacts, which will let you search your contact list for a new contact to add.
Add Manually, which will let you manually type in a first name, last name, and email address of a recipient.
Import from CSV, which will let you import a CSV email list to your campaign.