So, Too, Either, Neither

I understand this material pretty well. So and Too is used to agreement with positive senteces while Either and Neither is the otherwise.

Example using So :
Dina is happy
Tina is happy
Dina is happy, so does Tina

Example using Neither
Marry didn't go to the party
Nitta didn't go either
Marry didn't fo to the party, Neither did Nitta.