The Alliance option apears in the trade menu.
Some tips on improving relations:
-Give gifts (not too many now!)
-Improve diplomatic ability (this also makes your gifts more valuable)
-Don't go to war with them (kind of obvious)
-Trade with them (the more money your routes are making the happier they will be)
- Be the same ethical alignment
You don't have to research Xeno Ethics but it helps.
Also a good civilization can still ally with the Drengin
- Have a strong military (do you want make a powerful civilization angry?)
-Don't be evil (seriously it just doesn't work)
edit: posted 2 minutes after you