No easy feat as I see it. I guess you would need:
- custom widget which recognizes the @ and displays a list of possible suspects (don't know any in marketplace)
- on save process the mention
- user inbox or notification functionality for the user (e-mail module?)
- deeplink (module) to go from e-mail link directly to point of the tag
Still not easy, but maybe something like an onchange flow with settings “while entering data”: if ends with a ‘@’, show a popup with dataview object and inside a list of suspects with a select button that calls a flow with a change object action to append selected name/e-mail/identifier in the text attribute. And then process like Sjors suggested. Maybe you set an association in the select flow so you can retrieve users to be notified instead of parsing the string content.