How can I mark a new JIRA team member as starting on a given date?

[Paul MacKenzie]

We have a new JIRA user who is part of a Tempo team.  His timesheets for all of the weeks prior to starting as a new user are marked as 0 hours, and I would only like to see him in the timesheets for weeks he was part of the team, not the infinite time before that.

I did not have to add him directly as a new Tempo user, as the JIRA group in which he is a member is already in the Tempo team, so he's there already.  How can I mark his "starting date" so that he does not appear in previous timesheets before he got here?