When I'm new to a group, I feel a bit safer near the rear. This is especially true if the group rides as a paceline, i.e., each person follows the person in front of them pretty closely. If you're not used to riding closely behind someone's wheel or having someone closely behind yours, then hang near the back and watch what people do. In paceline riding, you need to make full and consistent use of hand and voice signals and need to keep a steady pace and spin. If the group is just riding together loosely, you still need to watch yourself and others, but not quite as closely. So, it really depends on how the group rides together.

In any event, put safety ahead of any concerns about getting dropped or embarrassed. Just do your best to keep up, but bear in mind that most of us have gotten dropped at some point or another. So long as you know where you're going, don't worry about it. And don't feel shy about admitting to, in the very least, the ride leader that you're new to this. It gives everyone an opportunity to welcome you and give you some pointers. You'll likely be pleasantly surprised at how gracious most people will be.