If using 1.3937) or 1.4(22) s/w the correct Avaya supported method is
1.Place the receptionist in all of the hunt groups , disable her if she is not to recive calls./
2. create a short code or amend the default code to represent.
Short code: *21
Tel no: leave blank
Feature: Set hunt group night service
and
ShortCode : *20
Tel no : Leave Blank
Feature: Clear night service
You can then create a toggled button on the users to set the reception group with this short code in and out of service, this will make all the groups that she is amember of in night service