@lee-martin
Yes, so unfortunately the built-in ability to do this is not going to be available. To make it happen, you need to employ the use of the Office 365 Users connector and have your combobox Items be a filter of the users returned from a search.
The trick then is that an Office 365 User record is different than a SharePoint user record, so it needs to be "translated" when written to your list.
Example for Items property:
Filter(Office365Users.SearchUserV2({searchTerm:Self.SearchText}).value, EndsWith(Mail, "co.uk"))
Then you will have a list of only users with the co.uk email addresses.
HOWEVER, this only deals with the list. Setting the Default item from your record and writing that back to the list is going to be dependent on how you are using this...is this in your Form, or do you have this in some other way?
