I'm one who finds it very useful to put contacts into groups. One thing I think that Messages could add that would be very cool is to allow users to add an entire group to a message. Great for a number of circumstances. You could add from searching or by adding a group directly (adding doesn't interfere with filtering feature). Could work similarly on OS X as well.
Sidenote: Mail on OS X allows you to add an entire group to a new email. Can't on iOS. Could be another great iOS addition as well.
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