Re: Can we use Constant Contact to send Welcome emails to users who sign-up to our service?
Yes, there is absolutely a way to trigger welcome emails when adding a contact to an account (like through a signup form). I've just updated our PHP example here to do so.
You just need to append your request URL when adding a contact to the account with action_by=ACTION_BY_VISITOR if you're using our V2 API. Keep in mind that the Welcome email will only be sent to an email address that has never been in the account before (so test with a new email address each time when testing the welcome email).