I have a beta hemolytic streptococcus, group b uti and the lab results indicate/suggest treatment with penicillin or ampicillin, but my doctor prescribed me 500 mg of amoxicillin for 7 days, twice daily. will this treatment be effective for group b?

The drug is concentrated/excreted in the urine and should be adequate to treat/kill all germs of a GBS in the urinary tract. The germ can live in the digestive tract (large intestine) and complete removal from that area may not be possible. However, its presence there is not considered an infection, its just one of many bacteria found in poop.

If you have no symptoms then this result is from contamination of the specimen. If you do- then amoxicillin should help

This isn't a common cause of UTI, but since it's often in the vagina, it causes UTI from time to time. If its in your urine at high concentration (usually 100,000 bacteria per ml), treatment is recommended, even without symptoms; it's potentially dangerous. You're on perfect treatment and the right dose: amoxicillin is nearly identical to ampicillin, just longer acting with less frequent dosing.