I agree with you./I couldn't agree more.

我同意/非常同意。

I'm with you on that.

我支持你。

I feel the same.

同感。

My thoughts exactly.

和我想的一样。

I'll second that.

我赞成。

I am on board.

我同意/我愿意加入。

I consent to that.

我同意/允许。

I quite approve of your idea.

我很赞成你的想法。