If you use Workflow|Messaging|New then in the resulting screen the To field looks like a plain text field, in fact it's a search field (if you type s and then the Enter key, you will get a list users starting with s).

Hence it should have a binoculars icon after it.

Note that I understand that it is a very special search field in that the search is individually processing each of the entries separated by semi-colons. But I still think a binoculars icon after it would immediatly convey that this is a search field.  [My initial reaction on looking at the screen to document it was "crap - one has to know the users names - you can't search for them". It's really rather sexy you can search and multiple entries and the user-groups are, by default, included in the search.]

Regards, Tim G