Yes, us is a Scrabble word!
...and is worth 2 points in Scrabble, and 3 points in Words with Friends
us
Noun
1. North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776
2. A base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine
3. A heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
4. The 21st letter of the Roman alphabet