Sometimes we need to mention someone to be aware of something, but usually, each task/to-do item has the responsible person, the assignee. It may be implicit or explicit. If you want to explicitly assign a person to an item, you can do this as follows:
Click on 'assignee' (avatar) icon, enter the username and select the corresponding user:
The Spreadsheet Editor table has a column named ‘Assignee'. If you want to assign someone, you may enter the username (@ sign is added automatically if not present)
As you may admit, the Spreadsheet contains pure text and pure ids, as we do not store or process any personal information.