I want to have a field that display user list when @ is typed and normal text in single user input field. Is there a component to accomplish this already?
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This sounds like a tagging type situation - currently there is not a single component that will combine both free form text and a user picker as you mention. There was a similar thread here recently which discusses parsing free-form-entered tags, but it does not match your requirement exactly.
However, using logic from the linked thread you could apply some different techniques, including a confirmation richTextDisplayField that parses the user-entered text, converting the @tags into user links, or within the validation parameter parse the tags and flag the user if any entered do not match valid usernames, etc.