I think that the question needs to be answered in several steps which is why I gave up on accomplishing the task of porting the module in the first place. It's a very useful Views field handler which is used throughout OpenAtrium.
The first question is how to convert these two lines of code from D6 api to D7 api since db_rewrite_sql() now has become hook_query_alter() and the $subquery and $main variables were constructed using some Views voodoo.
$subquery = db_rewrite_sql($subquery, $view->base_table, $view->base_field, array('view' => &$view));
$main = db_rewrite_sql($main, $view->base_table, $view->base_field, array('view' => &$view));
Of course, there are several other instances of the DB api that have changed which is why this is a complicated problem. But, yes, it is a big problem with many complicated parts so I'll just delete the original post.