Hello Will,
I just tried to reproduce the bug under Windows XP without success. Please make sure that there are users correctly added to that project. In order to do that:
1) Login to TeamTrick
2) Click on your project you are working on name on menu on the left
3) Click on Users & Roles on the menu on the left
If you see an empty list, just add some users and they will appear in the commitment creation form. Let me know if that was the case or not to keep looking for an alternative cause.
Thanks