To answer your first question, success rates for child/sex abuse survivor groups vary. The short answer is “it depends.” Overall they can be very helpful but it may depend on factors such as the competence of the group leader and whether or not the group is cohesive. It also depends on the individual attending the group. How ready are they to open up about the abuse? It may be too soon for some people. Other people may not like sharing personal information with a group. My suggestion would be to call local support group leaders and ask questions about their groups. Here’s a link that may help you locate local support groups.
The second question is more challenging to answer. There is no general answer. Each abuse situation is unique. I would need to have many more details about your situation to give you a specific answer.
If they continue to accuse you of something that you are not guilty of then it is in your best interest to stop associating with them. This would mean not visiting them, not taking their phone calls, and so forth. You may have to put an end to all communication with them for your own self-preservation. You cannot control the behavior of other people but you have control over how you react to them.
If you feel physically threatened by them then go to the police. They can advise you about whether a protective order would be the correct course of action. I do not believe that a protective order would stop them from spreading lies about you but if you feel physically threatened by them, it could help.
I hope that my answer is helpful. If not, consider writing back and providing more details about your situation. Information that would be helpful includes (but not limited to): what specifically are the accusations? Are they doing this in-person or are they calling you and leaving you nasty voice messages, etc.? Who are they making the allegations to? How often does this occur? Are they harassing you? The more information you can provide the better. Knowing the answers to these aforementioned questions may help me to provide you with a more complete answer.
Thanks for your question. Please take care.