がSubject marking particle, Identification particle

が is considered to be the subject marker in Japanese. This means that it marks someone or something that is performing an action, or it identifies someone or something as having a specific feature (this is usually a feature that the listener is not aware of).

In both of these sentences, が puts focus on the thing performing an action/existing in some way (as opposed to any other thing). Because of this, が is usually said to highlight what comes before it, while は highlights what comes after it.

が always shows the 'target' of a statement, when other targets also exist. This is why が is used to describe situations that are happening either 'in the moment', or 'individually of other things'.

Because the group in this sentence is only one group out of many possible 'groups of people' that could be described, が will be used.

Caution

が is not usually used to describe something about yourself, unless you are intentionally describing yourself compared to other people. If you want to talk about yourself 'in general terms', は will sound more natural (there are exceptions).