"The Communist Party USA (CPUSA) has been receiving membership requests ever since Donald Trump was elected President."
I'm not surprised. I'd also not be surprised if they had been receiving membership requests before Trump. As the population grows, and as new generations come of age, one would expect new membership requests in pretty much all parties to increase.
Now, if one wants to make an argument for causation, they would at minimum need to show that the rate of change has increased (not that that is anywhere near enough), but they do not. As a matter of fact, the only number they do present is a CPUSA membership of 5,000, or about 0.00152%, which tells me that even if their growth rate has skyrocketed since Trump, and even if Trump is 100% of the reason, the total effect is about jack.