throw in with (one)
To join one's activity or enterprise; to become associated with one. You're bound to get into trouble with the law if you throw in with that shady bunch. I've got capital to invest if you let me throw in with you on this.
throw in with someone
to join with someone; to join someone's enterprise. I will throw in with you and we can all go hunting together. Do you mind if I throw in with you?