Single contacts can only be manually added to static lists. To add a contact to your static list:
- Navigate in HubSpot Marketing to Contacts > All Contacts.
- Click on the name of the contact you'd like to add to a static list.
- Scroll down until you see List Memberships on the left-hand side, then click Manage list memberships.
- From Add Contact to a list dropdown, select the name of the Static List to which you would like to add the contact, and click Add Contact to this list when finished.
Adding a single contact to a Smart List would require changing the criteria of that smart list. If you only need to add one specific contact to a smart list, you can add an OR criterion and set it to Contact property > Email > is equal to [that specific contact's email address].