How about a drop down box to select a user to send it to as opposed to writing the username each time. And a send to group function (send to all users) - Maybe not for the general community as the spam risks posed are high, but could be a useful admin feature for you.

I've considered the mass user option for admin's.

Now, for the drop-down menu, as the site grows would you already want to to select a username from a list of 225?