What is _conveyor?

This is a "behind the scenes" user created by MakerBot's software - under Mavericks it didn't show up on the login screen, but under Yosemite it does. For whatever it's worth, you system has several of these background processes running; they're called "daemons".

I've found that uninstalling the previous MakerBot software and installing a current copy solved the problem (for me, under 10.10.1).

To uninstall and reinstall Makerware manually on the Mac:

1. In your Applications folder, drag the old MakerWare app to the Trash.
2. Open your hard drive icon (e.g., “Macintosh HD”), then the Library folder.
3. In the Library folder, locate the MakerBot folder and drag it to the Trash.
4. Still in the Library folder, locate the LaunchDaemons folder and delete this document: com.makerbot.conveyor.plist
5. Restart the Mac to kill off the running “conveyor-svc” daemon that’s still using the plist file you just trashed.
6. After the Mac restarts, empty the Trash.
7. Download, install, and launch the new MakerWare. The download comes with a MakerWare uninstaller app, so save that uninstaller and use it next time.

Hope this helps.

That's not a built-in user. If you didn't create it yourself, it was created by some software that you installed, or by someone else using the computer.