I took my son to play group when he was 2mths old. but it was the kinda play group where a bunch of moms gather together and hv their kids play together. it doesn't cost a dime and since we meet every week our kids r closed together. now that they r 9 mths old they recognize each other and to be honest, I think its beneficial to take the kids out more so they r not so anxious with strangers. and since we r a closed group and we all know that u dnt bring a sick kid to playgroup so far my son has not gotten sick after a play date. we go to each others house or to the park or to a kids cafe... a variety of stuff that they can do. of cos before they could sit up they couldn't hv much interactions with each other. but they knew each other by then and now its cute to watch them play together.
im a ftm so its important to me that I expose my son to different stimulation. if u ask me what he's learnt I can tell u now that he is 9 mths old when he sees some kids do sthg he would try to copy. he always eat better when he's surrounded by others. and they learn abt the existance of each other. which is important so later on we can show them how to share and be around each other. I wouldn't pay to get in play groups though. all u need is to put the babies in one room and u found urself a play date. haha