Sure: Count me in = i want to join the group, for whatever activity you are going to do with the group. Count me on doesn’t have any meaning

As the other two have said: count me in means to include me in your plans. count me on doesn't mean anything, however, if you change the word order - count on me, then it means that you can rely upon me/that you can trust me to do what I say I'll do.

Count me in This idiom is very common. It means I am joining that activity. Count on me This idiom is very common. It means I will 100% be successful doing what you need. Count me on This is not an idiom. There are ways to use this grammar to make a similar meaning, but this would be more difficult and incomplete. There is a different idiom that looks similar. Count me _________ You can put most words in the blank. It usually means the same as this. I will be ________ Count me finished. I will be finished. Count me gone. I will be gone. Count me ready. I will be ready. Count me on. I will be on(something like a bus etc).

“Count me in”... means that the speaker would like to participate in whatever activity is being discussed.... count me on ( is not an English phrase ).... but “count ON me” is a legitimate English phrase it means that the speaker is going to dependable”... and the person being spoken to can trust that if they needs something done.When one states that “count on me”

My pleasure 🙏🎶🌈