The expression I’M WITH… (also with other pronouns than I) is used to say that you agree with someone or support someone’s argument:
I get your point, but I’m with Jenny.
I think you were wrong the other day, but I’m totally with you on this one.
I need your help. Are you with me?
He’s always with his wife, even if she says something stupid.
– What do you think about my idea?
– I’m totally with you.
Here’s the video version of the article: