From any folder, select the "Folder contacts" section (sometimes labeled "Contacts") and click "Assign New Contact".
When the contact list opens, you can find the contact you need by sorting by any of the columns (Name, Company, Email, or Type), or you can use the search field.
To add them to the active folder:
- Select the contact(s) you want to include
- Click "Add Selected Contacts"
If the contact is not already in the system, you can click "Create new Contact" to add them.