As a play group you won't need insurance or to rent a facility either. You're not offering daycare or childcare so it isn't applicable to you. People that join do so "at their own risk" as it were and take responsibility for their own actions and children.
* Advertise about your group in your neighborhood, church or even local gym or kids things that your children are in to let people know about it.
* come up with a name for your group that will get people's attention in a positive way and that says who you are.
* some play groups offer a snack at each venue or a craft once a month. Decide if you want to do a rotating snack schedule with members or if you want to charge a flat membership fee for a year to cover snacks and craft items if you choose to do this. Most groups are between $20 - $30 per year to join for a mom and a child. Make someone you trust treasurer to keep track of funds and how much is spent. Some groups are a 100% free to belong system and if admissions or small fees are required for participation they mark it as such on the announcement or group calendar so people know in advance.
* Once you get people interested in groups ask which days they are most available and if that is morning or afternoon. Plan your activities and group things around times when the majority of people can attend.
* Alot of play groups require that members must attend a certain number of venues each month to remain a member. One's around me require 2 venues per month to stay an active member and to remain on the membership register.
* set rules for your group as to what is or isn't going to be allowed up front. Make parents responsible for the behavior of their children. Make an age limit for the minimum age of child your group can accomodate to the oldest. It is a good idea to state that "mom must attend with the child as this is not a sitting service" type statement. If you want your group to be focused around a particular interest or religious faith you need to state that as well in your membership information. It's also a good idea to state upfront to members your chain of command in the group for leadership purposes as well as how unruly children are to be dealt with during the group and what will constitute reasons for a member to be asked to leave the group membership.
* decide if you want to be year round or just during the school year from Sept. - May with a group hiatus in the summer months June-Aug.
* it is also a good idea to come up with a way to meet potential new moms for the group BEFORE placing them on the membership list. In this day and age you can not be too careful. Make sure that someone who is interested in the group is normal and an actual mom. Maybe arrange a few "new members lunches" at a local restaraunt where you and other helper mom's can meet these new mom's face to face since this is a group for children as well. You don't want someone that screams "pedophile" from their behavior or someone who is hostile in nature around children or other people for safety reasons as well as group harmony.
* scout out community locations and offerings to start your group. Barnes and Noble does free story times, so do libraries. Look for safe parks that would make nice picnic or play meeting areas for young children. Offer a weekly walking group at a park or neighborhood for mom's who want to walk and maybe a monthly scrapbook night for mom's into that. Consider having a once a month "mom's night out" for just the mom's to go to dinner or a movie together without the kids. If you have bounce places near you ask if you could book the place for the group for a couple hours and if they offer a group discount for groups of a certain number or more. Things that will appeal to larger numbers of people will attract more people to your group.