see http://www.bugzilla.org/docs/2.20/html/userpreferences.html:
"Users to watch". When you enter one or more comma-delineated user accounts (usually email addresses) into the text entry box, you will receive a copy of all the bugmail those users are sent (security settings permitting). This powerful functionality enables seamless transitions as developers change projects or users go on holiday.

In addition this allows managers/leads to be aware of all traffic related/waiting on their reports.

JIRA doesn't allow this, which is a shame.

leads can get notification based on component leadership, or various other notification mechanisms, but nothing that allows monitoring all traffic another user gets.

A clunky workaround is to create a filter for bugs reported/assigned to a group and subscribe to that report. if by RSS this will work, although many people still like email instead of rss...
If by email subscription by default the full filter results are sent, not the deltas since the last invocation. The only workaround here is to set the filter to only show differences in the period of time equal to the subscription frequency. As I said, clunky.

It will be much better to be able to define a list of users/groups you are watching, or at least to define the "lead" for each user so another user can get all their JIRA email traffic.