There are two ways to pragmatically add email addresses to your Constant Contact account, as for which is going to work best for you depends on how you want to are wanting this to function.
1) Add a single contact to your account. This would be the way to upload a single address at a time. If you wanted to programmatically add the contact to your account as soon as you received confirmation that it had signed up to your gmail apps email address, this would be the way to do that. This involves sending your contact details in XML by making an http post to the Contacts collection.
2) Creating an add/remove contacts bulk activity. If you're looking to gather up a number of email addresses and add them all at once, you would want to look at this option. This would allow you to make an application/x-www-form-urlencoded or multipart/form-data http post to the Activities Collection.
I hope this helps point you in the right direction. If you have any other questions or concerns please let us know. Thanks.
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