This idea can work with any field in the document. You can also use an advanced search dialog box instead of a listbox or multibox. This kind of search expression works within the existing selections – if you had already selected only the part of your company operating in Mexico then your expression will select a random twenty percent of employees in Mexico. If you already have some employee names selected then you might want to clear the selection before using the expression. And, like I wrote in my previous blog posting, you can save an expression like this in a bookmark.
(See the comments section below for a suggestion about how to do a random sample selection as the Calculated Dimension in a Chart)