** Using WeBWorK **

Sorry for not responding to this sooner; I didn't notice it until now. Here's a partial answer that has been working for me: instead of using $db->addUserSet(), I use WeBWorK::ContentGenerator::Instructor->assignSetsToUsers(). I should caution that this has worked up to WeBWorK 2.7, but I haven't tested it on 2.10 yet. My code looks something like the following:

  # $db is the database object
  # $ce is the course environment object
  my $fr = new FakeRequest($db, $ce);
  my $instrCG = WeBWorK::ContentGenerator::Instructor->new($fr);
  $instrCG->assignSetsToUsers( $setNamesRef, [@userIDs] );

  # FakeRequest is a shell that has the 
  # methods that the content generator expects
  # of a Request object:
  package FakeRequest;
  sub new {
    my ( $class, $db, $ce ) = @_;
    my $authz = new FakeAuthz;
    return( bless( { ce=>$ce, db=>$db, authz=>$authz }, $class );
  }
  sub ce { my $self = shift(); return $self->{ce}; }
  sub db { my $self = shift(); return $self->{db}; }
  sub authz { my $self = shift(); return $self->{authz}; }
  sub param {
    my $self = shift();
    $self->{$_[0]} = $_[1] if ( @_ > 1 );
    return $self->{$_[0]};
  }
  1;
  package FakeAuthz;
  sub new {
    my $class = shift();
    return( bless( { }, $class ) );
  }
  sub hasPermissions {
    return 1;
  }
  1;

Here $setNamesRef is an array reference giving the list of set names to assign to the array of users @userIDs.

In any case, in case it's useful.

Cheers,
Gavin