T.,
I belong to a few playgroups, and it can be hard to feel like part of the group at the beginning. Those Moms do know each other, and while they're not leaving you out intentionally, they look forward to some adult conversation, just like you do. They gravitater toward their friends, and chat. It's really not on purpose. My advice to you is to try and show up on time, in order to be onbe of the first Moms there. Moms are notoriously late for stuff like this, just because we have kids and they make us late for stuff. If you are one of the first there, then you can try starting a conversation with the next couple of women who show up, instead of trying to jump in when they're already settled in and talking.
A lot of times, it's easiest to ask about their kids. Start with how old is he/she? Then you can talk about what they're doing at that age, all of the cool stuff and annoying stuff they do at that stage, etc. We all love to talk about our kids, and can do it endlessly. You can ask them questions about finding a good pediatrician, where is a good park, etc.
Try getting to talk to a pregnant Mom. We've all done that part, so it's automatically an experience in common. It's amazing just how much detail a woman will go into with a stranger when talking about morning sickness, labor, and delivery.
Good luck, T.. It's awkard the first few times, but it won't take long to be one of the girls. Then, when you are, try to remember what this part felt like, and be the first one to talk to the next new Mom who joins. She'll really appreciate it.
Jess