If you have already created some Tags for your contacts, your tag entries will be displayed in the People Tab and Person View by tapping the Tags shortcut.
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Here is the Edit Tag screen where you can Select or Start Typing to add a new tag
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Here, there are additional options like Emailing and Messaging your contacts. This will open an email or text thread with all of your tagged contacts included.

You can also edit the name of the tag, remove contacts from the Tag by selecting the checkmarks, or delete the Tag entirely.