I am trying to regrout my shower stall and
tub surround, but the builder abutted the
tiles in the corners and left no room for the
grout. What should I do in the corners?

If you have no problems with water leakage, I would do what the builders did- nothing. You could also opt to squeeze a thin layer of clear silicone along the seam, however it may or may not be noticeable depending on your application. The butting is a common practice for some builders, it saves time and money.