You shouldn't care, it was a very good effort. You will care since you rarely do it, but you'll do better next time because of it.
There is really no way to work around a 'no' once she refuses. Once the no is verbalized it's time to move on. But hey just remember, she's just a waitress at a bar. Plenty of other girls out there and be happy you had the balls to talk with her (Which most guys can't)
To answer your other question, here's what I would of done differently (I actually hate doing hypothetical situations, but for the sake of learning)
first off, I would of just relaxed and enjoyed the time with my co-workers, even more so than usual If I noticed the cute waitress. Because when she sees the fun that is going on in my presence, who wouldn't want to be having fun with me instead of working?
I would ignore the waitress for the entire meal, barely notice she was there, and continuing focusing on having a great time with my co-workers. At the very end I would pull her aside and be direct and to the point(She's on the clock after all, you have to move fast) , saying something like:
Quote:
"Hey, I'm heading out but I thought you looked kind of cute. You might be a fun person to hang out with, so let's go out for drinks later. I'd like to get to know you."
with a confident smile.
Again, don't worry too much about the past. It's already happened and you can't change it. Just focus on what you can do right now and you'll keep getting better.