The expression "I'm in" or "count me in" mean that you wish to be included in a proposed activity.
"I'm going to the bar. Anyone else coming?"
"Count me in!"
I believe the expression may originate from gambling, possibly poker, or some other card game where players are dealt a hand and then decide whether they are playing on by saying that they are either "in" or "out". I'm unsure (sorry, I'm not really a gambler) but there may be other similar gambling expressions which mean that you wish to leave the game and for your winnings to be counted.
In your specific quote a man and a woman are making an arrangement to go out to a bistro/restaurant. The joke is that one guy says he has "the hair of three men", which he says to mean his hair is three-times as thick as most other men's hair, but the woman alludes to a possible second meaning. Personally, I assumed the second meaning was that he actually possesses some hair taken from three other men! But in comments below it has been pointed out a woman may be put off by body hair, and that she is talking about this. So she then adds a condition or caveat to his going out, which is that he will only go with him if he was talking about the hair on his head by saying "As long as it's on your head - I'm in!"