@mentions create a reference to another Quip user or a link to another Quip document. When typing a message or writing a document, you can @mention any Quip user in your Contacts by typing the @ symbol and using the typeahead. You can follow these same steps to create a link to any document that you have access to.
From your browser, you can also hover over a person's name to show their email address and the last time they accessed Quip, and send them a message directly from the document.
On a device, type the @ symbol, click on “Insert...” or click on the @ symbol.