How early someone can enter the U.S. depends mostly on their immigration status. This is something that should be taken into consideration when requesting start dates for CUA-sponsored non-immigrant statuses.

Nonimmigrant Status … …can enter U.S. ...
B-1/B-2 Visitor no earlier than the start date of their visitor visa

F-1 student

J-1 Exchange visitor

up to 30 days before start date on Form DS-2019

H-1B Temporary Worker

O-1 Temporary Worker

TN - Professional under NAFTA

up to 10 days before start date of approved petition