It depends on how you want to portray yourself.
Because the situation you are describing is 1) informal
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.
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