I'm looking for an addon that automatically says something like "Congratulations! :)" or something when someone in my guild unlocks achievement(s).

Thanks for your help in advance.

I'd suggest not getting such an addon -- we had someone in our guild using one, and it quickly became spammy.  If you do get one, please make sure that it has some sort of cooldown -- the worst spamminess was when people would do a guild run of an instance, and a bunch of people would get an achievement at the same time because of that.  We'd get:

Congratulations on your achievement, <name>!

five or ten times in a row!

And you know... how much is someone's congratulations worth if they can't be bothered to type it themselves?

:-)