Selecting an item from a large number of items is a very well-solved problem. However, there doesn't appear to be a good solution for selecting large numbers of items, ie. when the user needs to select 100+ items from a list of thousands. Note that there's no real way to reduce the number of selectable items - all those items need to be available to select at once.
My best approach solution so far is a list of checkboxes with a search field and "select all" and "un-select all" buttons. It seems like it'd work, but it feels clumsy and I'd love to know if anyone's got a better solution.
I think the Select2 control suggested by Danny Varod in that thread could be used in your context. It is very similar to what Facebook does to allow users to select friends to share content with. Hope it is helpful. :)