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。