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.