Adding contacts on SuperPhone happens in 4 ways:

What is your form url? For example, Ryan's is ryanleslie.superphone.io

The best way to add a single contact is just fill in that form on their behalf. Make sure you have their permission to send them SuperPhone messages.
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Keep in mind that when you add a contact via your form, they will get the form auto-response that you've programmed. To add contacts without triggering the form auto-response, use the contact import feature or add them via the mobile app.