What is this for? According to the mailing list
The people link will show up when there has been a build containing
changes in a scm, then the comitters for that build will show up in
the list. Before that (is done) the People link will not show up.
In other words, the usernames listed in the 'People' list will not necessarily be the names of Jenkins users (ie users who can login to the web interface) - the names are commiter usernames (or perhaps the email addresses with the domain stripped) as recorded in the commit logs.