I agree.
So do I.
Me too.
Me neither.
(Agreeing about a negative idea.)
I don’t either.
(Agreeing about a negative idea.)
You’re right.
That’s right.
Good idea.
I think that’s a good idea.
Disagreeing
I disagree.
I don’t think so.
(No.) That’s not right.
Yes, but...
(I’m sorry, but) I don’t agree.