Hi!
I like the idea of advertising for your own playgroup with a theme in mind! That's a good suggestion.
I'm finding, with a new group of friends that i have just formed (we're in a new town) that we don't have to agree 100% on parenting (that will NEVER happen) to get along and for our children to benefit from being together.
Maybe try attending a program at a community centre or library. Then you'll have the structure and you can "click" with the ones that you click with and form a friendship and start adding people on as you find that they click. There's a rhyme time class once a week, in this town, and just from attending that class I have met so many folks to get together with! The more structured programs seem to attract the moms interested in early education and being actively involved, and not those who are just interested in chatting (which has it's place too!). Maybe there's something similar where you live? Or try looking up a group like La Leche League and start attending some of their meetings for moms who nurse toddlers. You might have to informally start your own group that way -- just find individuals that you jive with and invite them on play dates or whatever. Sometimes individuals are more manageable than whole big groups anyhow...
I have formed an at home preschool with a friend of mine -- we have 7 kids between the two of us, and we're working with the oldest four as a preschool group. If we had other moms interested we'd definitely invite them to join. We're just starting out and want to really get our feet under us, though, before we start expanding!
Seven is already a houseful too!
Good luck. It's hard to find a group sometimes. I've had such difficulty in other places we've lived.