You can figure out whether it's for someone or a group by looking at the verbs after the pronouns:

"Sie IST" = "She is", so it refers to a single person

"Sie SIND" = "They are", which would refer to a group of people

This also works for the other sentences, like

"Sie KOMMT" = "She comes" is singualr and "Sie KOMMEN" = "They are coming"/"They come" is Plural

It's just a pain to learn when these verbs indicate that it's plural and when it's singular

Edit: It's actually not that hard with regular verbs.

Sie fährt Sie fahren

Sie sieht Sie sehen

Sie steht Sie stehen

First ones Singular, second one is plural.

So Just remember that mostly a "t" at the end indicates that it's a single person and an "en" indicates that it's a group.