Found this on United Airlines website

By printing your boarding pass through united.com, you can proceed directly to security. If you have bags to check, you may check-in online, and then use EasyCheck-in Curbside or an EasyCheck-in Kiosk at the airport.

In order to use EasyCheck-in Online, you must:

Have an electronic ticket
Be a Mileage Plus Member
Check in between 1 and 24 hours prior to your scheduled departure
Travel on a United, Ted or United Express flight
Travel within the U.S including Alaska, Hawaii, San Juan, Puerto Rico, or St. Thomas, U.S. Virgin Islands.
Travel in a group of 9 or less
Use a computer that is connected to a working printer