There is a MOPS group that meets here in McKinney at the First Baptist Church. There is also an organization called the Allen Early Childhood PTA in Allen that has lots of activities. If you can't find any info via internet on these let me know and I'll get you numbers.
If you are looking for a smaller group I would suggest getting into a child's class such as Kindermusik (they offer it through both the Allen and McKinney Parks and Rec's) or Gym such as Little Gym in Allen. Classes like these are a great way to meet other moms that have kids the same age to start your own little playgroup. That's the way we started our's for our older kids and we still get together 2+ years later!!