Mmm, let's see:
- autocomplete
- Google Suggest - Auto Suggest<

it is easy to create a MOD for that, but you need to consider what do you want to be complete textbox...
there is always to ways:

- an Offline, when the user page loads all the words are already in the code ready to be used, and it is the fastest
- an Online, where it uses AJAX technology to query the database using the word that the user just wrote, then get the results and write as a dropdown... this take time depending on the user connection and your forum / database, and per each letter there is a new query sent to the database.
if you want help... please let us know what do you want to accomplish.<

how good / comfy are you with ASP / HTML / Snitz?<

good...
here you have:

http://www.balexandre.com/snitz/autocomplete/

now all you need, is to apply a query to get all names/emails form the current user and format it accordingly

to test the emails you can see the code, to see what names can you search.
Hope that this give you the way to build what you need.<

Javascript is one of my weaker points.
In var emails below, how do I query the db and put the results into a list like below? I know ... talk about an Achilles heel.
var emails = [
{ name: "Peter Pan", to: "peter@pan.de" },
{ name: "Molly", to: "molly@yahoo.com" },
{ name: "Forneria Marconi", to: "live@japan.jp" },
{ name: "Master <em>Sync</em>", to: "205bw@samsung.com" },
{ name: "Dr. <strong>Tech</strong> de Log", to: "g15@logitech.com" },
{ name: "Don Corleone", to: "don@vegas.com" },
{ name: "Mc Chick", to: "info@donalds.org" },
{ name: "Donnie Darko", to: "dd@timeshift.info" },
{ name: "Quake The Net", to: "webmaster@quakenet.org" },
{ name: "Dr. Write", to: "write@writable.com" }<