Non Payment of Service Charge

am seeking your guidance regarding a non-payment issue with one of my former consulting partners. Here is a brief summary of the situation: I worked as a tax consultant for this unit, handling their client work for a reasonable fee. The unit, operational since 2021, ceased operations in September 2024. My service charges from July 2023 onwards remain unpaid despite multiple reminders via WhatsApp, emails, and a registered letter. Their response cited "billing irregularities" in my invoices without providing any specific details. I have now decided to file a legal suit to recover the unpaid amount. My question is whether it is advisable to include their clients—who are the ultimate beneficiaries of my work—as third parties in the case. I believe this might exert additional pressure on the non-paying consulting unit to resolve the matter. Could you please advise on: The feasibility and legality of including third parties (their clients) in the suit. Any potential risks or implications of such an approach.