The easiest way to do this is to have a dropdown like you said, but instead of placing the multiple users/values in there, instead make a javascript IF statement that will populate however many Single Line fields you need, JS says something like this:
IF dropdown = Business Office
FILL Person1_field = domain\Mary
FILL Person2_field = sam@domain.com
etc
Of course, you can make the Person1_field and Person2_fields hidden so no one can make changes but pay attention to choose to Save Data instead of Ignore.
Now for the code, assign the CSS of "dropdown" to your dropdown. Assign CSS of "user1" to your Person1_field, "user2" to your Person2_field....
$(".dropdown").change(function(){
var dropdown = $(".dropdown select").val();
if (dropdown == "Business Office"){
$(".user1 input").val("domain\Mary");
$(".user2 input").val("sam@domain.com");
}
else if (dropdown == "Human Resources"){
$(".user1 input").val("nikki@domain.com");
$(".user2 input").val("domain\paul");
}
});
Alternatively, you can do it with a database and a lookup, but the above solution is probably simpler if you aren't familiar with Databases.
Hope that helps