we have 7 users added, but only 4 appear on Wiki->Browse->People directory and can be added for team calendars events
Have all 7 users logged in yet? If not, get the other 3 to log in and then have a look :)
The people directory is only populated with users who have logged in at least once before.
Ultimately this is the result of the following bug:
The current workaround is to visit the following url, adjusting the <missing_user> to each username that isn't already showing up: