You need to be logged in into Xero organization to be able to get to project users.
Select from the menu Add-ons -> G-Accon for XERO -> Get Projects Data
Select Users and the fields you want to be pulled into Google Spreadsheet. The list of all user fields and descriptions can be found at Xero API documentation page.
User list can be filtered by user name and user email.
By default, the user list is sorted by user name. The user list can be sorted by the user email as well.
Xero user list example: