Twitter-style autocomplete in textarea
Have you tried this
GITHUB: https://github.com/podio/jquery-mentions-input
DEMO/CONFIG: http://podio.github.io/jquery-mentions-input/
It is pretty simple to implement.
Another great library which solves this problem At.js (deprecated)
Source
Demo
They are now suggesting the Tribute library
https://github.com/zurb/tribute
Example
I'm sure your problem is long since solved, but jquery-textcomplete looks like it would do the job.