Best method for selecting multiple checkbox items

I am developing an internal messaging application which is to be used by approximately 500 users. When composing a new message it will ask who 开发者_开发技巧you want to send the message to. Instead of presenting the user with a checkbox list of 500 users which they can pick from (i.e they may only wish to send to 20-30 of the 500 users) I wanted to ask if there was a more user friendly and/or more efficient way of doing this - without the user having to type the names of the recipients in an "auto suggest" way (like how FaceBook does it). The solution can rely on Jquery/Ajax functionality.

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Is there a common way to group people that would be getting emails, i.e. different departments? If so, you could break the checkboxes into multiple categories, and add checkboxes that say 'email all of marketing' for example, that would auto-select those people. This would make it easier to read and easier to use.

Additionally, you could add some functionality to allow the users to set up custom mailing lists, and then select from one of those when they want to email these people.

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Guys, this is what I came up with in the end;

http://pastebin.me/c46afb39cddf7945967e2a6f17078d52

It's not ideal but I couldn't think of anything else. You can toggle select all of the checkboxes or you can click and drag a selection to select multiple checkboxes in one go. It uses Jquery UI "selectable" to do this.