Our developer has an account and presumably an API key. We would like to add code to our asp.net pages so that when a visitor registers on our site we can insert to our CC list, instead of having to wait to export and then import. How does this work? Do we need our own key? Can we use the developer's key in our pages and still connect to our lists?
You can absolutely re-use the API key that was already created by your developer. With the API we require that you have an API key (Which identifies your application/integration) and an Access Token (which identifies your Constant Contact account). In order to get the Access Token, you (or your developer) can navigate to https://constantcontact.mashery.com/io-docs and copy the API key into the API Key box. Once you've done that, click on Get Access Token and simply follow the flow until you see the Access Token at the end.
Once you have the API Key and the Access Token, you can use our .NET SDK (which includes an example for how to add contacts) in your code to set it up so that it automatically creates these contacts in your account. You can find this SDK here: https://github.com/constantcontact/.net-sdk
If you (or your developer) have any questions, please let us know!