"person" is the singular form, as in : There is a person here.
"people" is the plural form, as in: There are people here.
Words such as "orchestra", l which may have many people, is singular, such as: The orchestra plays (3rd person singular) well.
"Yes! There are" is the correct response. This is because you are describing that there are many people on the street. The word many is plural, so you would say are. If you want to use the word is, you could say "Yes, there is someone."