I'm new to constant contact and recently signed up for a business partner account. We have set up two clients and we need a basic API to send over to our clients' webmasters to integrate into their site. This API should include a subscribe/unsubscribe option as well as a sign up form for their homepages. Also, some sample content to paste on their subscribption webpage would be helpful.
If you are looking for a simple sign up form, you can certainly use the "Join My Mailing List" form that Constant Contact provides you. You can get this code by clicking on "Contacts > Join My Mailing List > Start Wizard to Generate HTML Code". Each sign up box will need to be created from each account, as the HTML form that is generated will be tied to that specific account.
If you are looking for a more customized form, you take a look at the C# and PHP Sample forms that we have available here. If you have any other questions, please let us know some more details of what you are looking to do and what programming language you are looking to use.