Is all I need to do to be a utilitarian is attach any amount of utility to other peoples' utility function and/or feelings?

Attach amounts of utility to possible states of the world. Otherwise no constraints. It is how utilitarianism is probably understood by most people here. Outside LessWrong, different definitions may be predominant.

"But 0 funU is really, really bad -- you're just sticking the really bad mark at -3500 while I'm sticking it at zero."

As you wish: so really happy 3600, happy 3570, sad 3500, tortured 0. Utility functions should be invariant with respect to additive or multiplicative constants. (Any monotonous transformation may work if done for the whole your utility function, but not for parts you are going to sum.) I was objecting to relative differences - in your original setting, assuming additivity (not wrong per se), moving one person from sad to very happy would balance moving two other people from sad to tortured. That seems obviously wrong.