How do I add a user and allow them access to only one issue?

[Jeremy Gaudet]

Not easily.  You'd effectively have to have a security level defined in the project's Issue Security that is specific to that issue, apply that level to that issue (and apply the 'normal' level to all other issues), then add the new user to that issue specific security level.  That's high level; I've never used issue security, but that's the only way I can think of... you can't do it using normal role-based privs, as those apply to all issues in the project.

[Nic Brough -Adaptavist-]

FWIW, Jeremy is absolutely right about the Security Scheme stuff (as well as everything else that he was already confident about)