Originally Posted by United Hub
Group 2 – Premier Access boarding
o Premier® Gold
o Star Alliance™ Gold
o Premier® Silver
o MileagePlus® Club Card members
o Presidential PlusSM Card members
o MileagePlus® Explorer Card members
o MileagePlus® Awards Card members
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.
So the official answer is no - if you are just *S and not MP Silver, you don't board with group 2. Whether this is enforced on a gate-to-gate basis is another story entirely.