Children can find it hard to talk/interact appropriately within their peer group at this age. My DS is a motormouth and hardly stops to breath but in a group of children he is as quiet as a mouse.
Around the age of 2 (sometimes earlier, sometimes later) there is often a language explosion when they go from a few words to a whole heap within a short space of time.
It is suggested they should have around 20 words by 2, but more important than that is that they show that they understand what you are saying to them.