If you OR YOUR PARENT sold some of your rights then you likely DO NOT own "exclusive rights". You might still own some "non-exclusive" rights. You might own NO rights. Perhaps the sale is voidable or void, although not likely. Perhaps you OR YOUR PARENT "licensed" rather than "sold" rights, in which case perhaps you have grounds to terminate the license. You do have a copy of the written license agreement or written assignment agreement for this "purchase" don't you? If not you need to get a copy. If this was some verbal deal, you have a huge legal mess to unravel, hopelessly beyond your abilities but well within the skill of a good IP attorney.
If you are now unhappy with the deal and want to change it, avoid it or cancel it, then you must in all prudence get an attorney to help you do that. An attorney needs to see the Agreement, hopefully written, by which you did whatever deal you or your famous parent did. Your terminology leads me to wonder what you OR YOUR PARENT actually sold, since you apparently think you gave a non-exclusive license ("non-exclusive rights") but use words that sound like a limited assignment. Find an attorney that knows this area, likely an Intellectual Property law attorney or Entertainment Attorney (if your famous parent was an entertainer), of the law to advise you. This is way beyond what you can or should handle yourself. So, the specific answer to your specific question of "What can I do?" is YOU CAN GET AN ATTORNEY.
Avvo, here, is a good way to find an attorney. Either pick one from those answering (to show you their skill and knowledge is why most that answer here are providing these answers) or use the Find A Lawyer tab. Ask for a free initial consultation so you and the attorney you select can evaluate each other to determine if you like the attorney and if the attorney is interested in your case and thinks you will be a good paying client.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Q: "They purchased non-exclusive rights from me for a nominal fee. Since that occurred they have used the rights and led millions to believe they own the rights exclusively."
R: The only way for an attorney to assist you is to first evaluate that agreement. The results of that analysis will determine the following steps [if any]. Which you must have already known. So speak with an attorney licensed to practice in the state whose law controls the agreement you made with the "group of individuals." The agreement should clearly designate that law in a provision entitled "choice of law." Good luck.
The above response is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.