Is there any way around this using the API? If I send a marketing blast promoting my email newsletter site I could easily have more than 5 people within 30 minutes create an account.
Yes, that is the limit we have in place, and in our experience it has not ever impacted someone legitimately setting up new clients, whereas we do have people accidentally or intentionally hammering the API and need to be careful to protect our customers.
If that is a real risk for you (and it could be in some circumstances), we'd suggest you just let people sign up to a list, or your own form, where you can collect them and add them later.
I have a very legitimate reason I need this! We have a group that represents over 750 independent grocery retailers and we are going to manage websites for them. Went to generate the websites the day before the launch and this limit is holding things up.
Is there any reason you need an entirely new client for each person who signs up in your application, and why you couldn't use a separate list for each signup belonging to the same client in your account?
Your answer to this will depend on which features of Campaign Monitor you want to use for each of the people who sign up in your application.