Note that it depends on the AD Cmdlets from Quest, and that it works in much the same way as the example. Whenever it finds that one of the group members is a group, it calls itself on that group and adds the result to the list of users.
Also note the technique of using a hashtable to keep a list of unique users. It's common to store a list in an array and use use -contains and -notcontains to find out if an item already exists, but this is tremendously inefficient compared to using a hashtable as an index, especially as the list of users gets longer, because it has to keep cycling through the list over and over. It's a trick I picked up in Perl that works equally well here.