The correct answers are I'm definitely up for it! Count me in. and Sounds great! I'm planning to join you guys.

"Count me in" is the correct phrasal verb to say you want to participate. "Count on me" means "rely on me," but "count me on" is incorrect.

The verb "plan" can be followed by a to-infinitive (planning to join) or "on" + gerund (planning on joining). Using "on" with a base verb is grammatically incorrect.