I have everything set now so the computer just needs booted and when it's done the only thing I have to do is add the user as a local administrator. Is there a way to do this automatically by entering something in the .sif?

If you're running an AD domain, just make a group, add the user to the group. Add the group to the workstation's admins group while building your image.

I plan on doing this for many users and dont want users to have local admin rights to computers that arent theres.

Create the appropriate OU's and use GPO's at the OU level to put domain level security groups into the local groups. I do this with the executive level employees. That way they're admin level for all executive computers.. in other words they can screw up their own computer or other executive computers - but nobody else's.