Check with the manager of local children's clothing stores or enroll your child in a group such as Gymboree or Kindermusik and meet other mother's there. If there is not a group in progress in your area, check with your SAHM friends to see if they are interested in getting together on a regular basis. If you don't know any other SAHM's strike up a conversation with mom's that you see at the mall, park, or Drs. office. Chances are, if they are there during office hours, they aren't working during the day and would probably love the opportunity to meet other SAHM's. Once you have found interested moms and kids then you must decide as a group how to organize your club.
Some groups are very informal and some are quite structured.
The larger the group, the more organization is needed to make things run smoothly. Informal groups usually meet in the homes of the members. Of course you would limit the number of children in each playgroup to no more than 4-6 in a home setting. More than that can bring more chaos than fun and the last thing a SAHM needs is more housework after her house has been turned upside down by a large group of children. Decide on a day and time to meet.