Issue : how to @mention someone who's not in the current project/group

In issues, it seems we can only @mention someone who’s a member of the current group or project.
In my workflow I would find interesting to be able to ping anyone from an issue description/comment that I am reporting. Is it possible ? Is there an option/permission for it ?

Another great feature to have (but that’s another subject) is the possibility to set the reporter manually when creating the issue.