We have the Pro 5 user addition.  It came with 5 licenses.  You can have as many users as you want to access it but only 5 at a time at one time.  If there will be more in it at one time you will need more licenses.  We like it because we can all be on at the same time accessing the server and everything gets saved immediately so the other person can see what we just did.
But we have 2004 version and there was a printing problem with using a server and printing remotely.  But I have been told by Inuit that the 2005 version corrected this problem.  I just don't want to go spend $800 to get it just yet.