Limiting Choices in a Person Field in PowerApps

I am trying to create a PowerApp that includes a Person field. I would like to limit the choice of people to a subset of all our users. When the person using the app chooses a user, I would like for it to show the person's name and picture.  So far in my attempts, I either get no suggestions as the user starts typing a name, or they see everyone in the O365.

So

1) What is the best way to define this subset? A SharePoint list with a person field? A SharePoint site security group?

2) How do I tie this subset to a combo box so that only these users are available for choosing.

Thanks.