This happens to me rather often: it is not always easy to enter group conversation from the first attempt and in appropriate manner. I usually listen to conversation a while, receive topic and emotional context (it is even more important in usual chit-chatting), then try to reply somebody or insert my own answer. Sometimes several attempts are required, but I think, it is rather common thing even for NT, they just can do it more elegant, less annoying and more "automatic"
P.S. Sometimes not being able to enter group conversation may mean, that they are not wanting you enter it. It may be understood by short and emotionally cold or negative answers.