It depends on how you want to portray yourself.

Because the conversational situation you are describing is  
1) informal (someone you know well)  
2) you are being invited to a party  
3) you are talking to a co-worker, even though it's your boss  
4) assuming you are on good terms with your boss

using
>I'm in

is appropriate, of course adding "thank you" is courteous
>Thanks for the invite, I'm in!

would convey a certain excitement of looking forward to going to the party. If you are too formal you may come across as being aloof or distant.