User mentions are now available in Spreadsheet and usable the same way as in Confluence.
Start Typing '@' to display the user searcher. Then type part of a user name to filter results.
From the user mention popup, you can navigate between results using the mouse or the keys arrow down/up. Hit Enter or left click to select an user.
The render is the same as Confluence in edit or display mode.
Spreadsheet sort/filters can be applied on user mention.