From here, you can search for existing contacts in the search bar, and use the Manage icons to update Contact Details for specific people. You can include a name, email address, photo, and other notes in the Contact Details. If you're on Textline's Standard plan or above, you also have the option to create your own Custom Fields for your contacts, so you can include whatever information is relevant for your company.
You can add individual contacts manually to your Address Book by clicking the blue New Contact button, or perform a mass import of an existing contact database using a CSV file, for which you can download the template here.
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