When submitting a comment to an idea, the comment author can tag other users to include them in the conversation. To tag a user, simply type @ immediately followed by the tagged user's name.
When tagging users in the comments of an idea, you have to type the first and last name with no spaces in order for the names to appear. If there is a space inserted, the tagging function takes that as a break.
Please note that currently, tagging users in expert review comments is not supported.