From this thread

http://www.flyertalk.com/forum/unite...ay-2013-a.html

Effective May, 2013:

Pre-Boarding

Customers with disabilities, then Global Services and uniformed military personnel

Premium Access Boarding
Group 1: Global Services, Premier 1K, Premier Platinum, premium cabins
Group 2: Premier Gold, Star Gold, Premier Silver, MileagePlus Presidential Plus and Club cardholders, MileagePlus Explorer and Awards cardholders

Note: A Star Alliance Silver member who is also a MileagePlus Premier Silver member traveling on United is able to board in Group 2 with a companion, as part of the MileagePlus Premier Silver benefit. This benefit does not apply to travelers who are Star Alliance Silver members from another airline.

General boarding via WilMA (Window Seats, then Middle Seats, then Aisle)
Group 3 - Window Seats
Group 4 - Middle Seats*
Group 5 - Aisle Seats

*Group 4 will be skipped on UA Express flights (as there is no middle seat).

Tip: If you’re traveling with a companion and one of you has a higher boarding status, you both may board with the earlier group.