Making a single issue visible to someone who isn't in a group w/ permissions

Our security issues are configured to only be visible to a single group (i.e. the security team).  At times that team wants to draw from expertise from outside their group and pull in individuals to look into a bug, w/ out adding them to the security group to make all the other bugs visible.  Is there a simple way to do that (similar to the concept of cc on Google Project Hosting)?

We've tried:

• Staring the the user's behalf
• Creating a field that adds users

Neither of which worked.

Many thanks in advance.