You have to be breast feeding at least 6 weeks.
You are given extra "points" to eat - and they give you an extra booklet to read about it.

I don't think you have to go to meetings... if you join up online. You would just mention you're breastfeeding, and they will send the extra booklet too.

Weight watchers isn't a 'diet'. It's a programme that teaches you to eat healthy food. No shakes, no pills. Just regular food - in the right quantities. And encourages you to do some exercise if you can - even if its just a walk. By writing your food intake down, or counting points - it makes you aware of how much food (and what type of food) you are actually putting in your mouth.

Weight watchers is about education, awareness and support.
It's quite safe to do it after the 6th week - so if you do - good on you and good luck with it.