“Nah, I’m good” is a full sentence. You don’t owe the group a TED talk on why you’re not doing it.

A reason is a door. The moment you explain why you are not doing something, the explanation becomes the thing to argue with, and you end up defending a case you never had to open. Declining is complete on its own. People who push past a short no were never going to be satisfied by a long one.

Practice the short version somewhere low stakes, then let the silence after it sit. You will feel the pull to fill that silence with reasons, and you can let it go by. If someone asks why, saying you would rather not is also a full sentence.