She has good focus. Just because she's playing alone doesn't mean that she's unaware of the excitement around her. Plus, the capacity to block out the noise and concentrate on her preferred activity is pretty impressive!
It's too early to "pretend." In general, young toddlers do not play coordinated games with themes, like "house" or "Batman." They engage mostly in parallel play, which means they work with toys in the vicinity of their peers but have little real interaction.
Toddlers can be shy, too. Just like grown-ups, toddlers are more likely to get chatty after getting to know someone first. As they start to become more comfortable with one another, they may begin to play imitative games together. Usually this involves Toddler A doing something silly, and Toddler B copying it (add a dose of laughter and repeat).
As long as your toddler is happy to go to playgroup (and you are, too), you can be certain that she's getting something out of it. Even if the group ends without your silent sweetie ever going near one of her peers, she has learned from the experience. Watching them play and observing as you interact with the other parents will help her when she begins her own social interactions. However, there is no research to indicate that playgroups are essential to development, social or otherwise, so don't let playgroup pressure get to you.