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:

Office365Users.SearchUser()

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.

  

Left(
Concat(ComboBox1.SelectedItems,Mail&”;”),
Len(Concat(ComboBox1.SelectedItems,Mail&”;”))-1
)

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.