Apple's System Image Utility

Which type of NetInstall image are you trying to make? and what is your source?
Why your own .sh script, while there is an (advanced) workflow step do add users?

Best manual I know of is here : http://training.apple.com/pdf/mac_management_basics_10.10.pdf (it is for 10.10, SIU looks different in 10.11 and 10.12, but the advanced workflows work the same)

I think SIU is not as easy as it looks, (like Composer), but if you know the tricks it can create good NetInstall images.

Some steps of the workflow in advanced do not work if the source is an installed OS partition. (example: create users will fail while users exist on the source volume)
Starting from the Install OS X .app gives best results with all workflow steps. Starting from Installed OS partition works better if you have added a lot of software to the partition. Your choice.

I was trying to do a Netinstall. I have tried the built in one which Apple told me only works for netboot images. I always use the install mac os x from the app store for theses.

Have you tried the 'custom' button in SIU? that will open up Automator with actions.
there is a workflow 'add user' as show in this screenshot:

SIU is picky with OS versions, you need to run the SIU on same OS verion as the .nbi you try to create.
I am not sure if the workflow shown above will create a working NetInstall, it is supposed to work for NetRestore.

Which version macOS are on?

@kerickson really stuck with SIU? No AutoCasperNBI?

There is some issues with SIU for older Macs, it's currently generating a kernel cache to large for older Macs.

Also, "NetInstall" is the recovery environment... so has no user.

What is the user account for?

@bentoms Its used for the Local Admin account. Yes stuck with stupid SIU. Can AutoCasperNBI work without us owning anything in JAMF?