Firstly add a Office 365 users connector on left side of PowerApps.
Now Add Combobox to your canvas.
Select the items property of ComboBox and give this formula:
In data Select the layout as People
Now your Combobox will populate all the users.
Now try to select two users from ComboBox and assign to the label.
So drag a label to PowerApps canvas and in the text property of label give this formula.
or you can write alternate formula to combine
Concat(ComboBox1.SelectedItems, Mail & “;”)
Now Label will show you the result concatenated with two users seperated by a semi colon. Like below.
So this is how you can achieve.