How to set up issue permission for current assignee and the people who are in the same group

[Wayne]

My company works with an external web development agency and we'd like to set up JIRA issue permissions such that

1. External developers can only see issues assigned to them.
2. The external lead developer can see all the tickets assigned to the external developers.

Is there a way to set up permissions mentioned above?   We have configured the permission scheme to allow Browse Project permission for company's own user group and Current assignee.  This gets us the 1. mentioned above but not 2.   If there's no way to set up project like this, then we are okay with external developers being able to see what other external developers are working on.

[Mikael Sandberg]

It can be done by using issue security scheme, there you can control who can and cannot view issues.