Yes, ally is a Scrabble word!
...and is worth 7 points in Scrabble, and 8 points in Words with Friends
ally
Verb
1. Become an ally or associate, as by a treaty or marriage
Noun
1. A friendly nation
2. An associate who provides cooperation or assistance
What Scrabble words can I make with the letters in ally?
3 Letter Words
2 Letter Words