The best way to add an email address to your contacts is to ask the person you'd like to add to send you an email message.
Open the email in the Mail app. Explore near top of the screen for the email address of the sender. VoiceOver will speak "sender's name, address,". If you can't easily find the address then explore lower down the left of the screen to find any early part of the message and then swipe left to locate the address. Now triple tap. This brings up a menu through which you can swipe right or explore by sliding a finger over the menu. Swipe right to locate View Contact Card and double tap. In the contact card swipe right repeatedly and you'll find both "create new contact" and "add to existing contact". Locate "create new contact" and then double tap. This brings up a partially filled contact information form. The first name, last name and email address should already be filled in. Swipe to check them and make any necessary changes. Finally swipe back left to the "Done, button" or explore near top right to locate it and double tap.
That's it; you've added that person to your contacts.
If you aren't using VoiceOver, all you need do to get to the screen with create new contact or add to existing contact is to tap on the from address twice.
If it isn't possible to get the new contact to send you an email, then you can add a new contact in the Contacts app.
Open the Contacts app; you could ask Siri to "open contacts". As usual, the app will start wherever you last left it. You will need to navigate to the page which lists your contacts and which is headed "Contacts", or possibly a service name like "Gmail" or "iCloud". Locate the "add button" near the bottom right of the page. Double tapping the add button takes you to the add contact form. VoiceOver will speak something like "text field, is editing, first name". Enter the first name using the on-screen keyboard, dictation or any other input method. If you used dictation then you can swipe right to the next field, Last Name, and double tap to begin editing it. If you used the keyboard VoiceOver will now be focused on the keyboard. Explore the screen above the keyboard by sliding your finger over the screen to find the last name field and then double tap to begin editing the last name using the on-screen keyboard. Alternatively, find any field and then swipe left or right to locate the field you want to enter next and then double tap to begin editing it. Repeat this process until you have entered all the information you want to enter for this contact. Finally, locate the "Done" button near top right of the screen or panel and double tap to save your new contact.
If someone has sent you an email which includes the email address in its body or perhaps you have found the email address on a website Voiceover will sometimes pause just before the email address is spoken. Swipe right and you'll probably hear the email address spoken. If VoiceOver doesn't pause, try pausing VoiceOver speech with a two finger tap and then swipe left to find the email address. Alternatively, set the rotor to links and swipe down to locate the email address. This may be the only method which works for text messages. Once you have heard the address spoken, triple tap to reveal a menu and swipe right to locate the add to contacts button. This will take you to a partially completed contact. Complete and save with the Done button.