Simply install the GetResponse Joomla! 1.5, 1.6, 1.7, 2.5 and 3.0 Native plugin, give it the list name and your API key (provided by GetResponse), and all new Joomla! signups are then added to your GetResponse list as well.
It's lightning fast and extremely reliable - and of course, since you are doing things the "approved" way, your technical solution is 100% in compliance with GetResponse's policies.
Hooking into the core Joomla! registration process, the system works with all Joomla! native components, including Community Builder and VirtueMart.
UPDATE v5.9: updated for Joomla! 3.0
Once installed, there are only two fields to complete: your email list name and your API key which is supplied by GetResponse. That's all there is to it.
Then, fully expecting to see errors, I ran my test registration and I was blown away at how seamless this little baby works! I cannot say enough good about it - it just works and it did not interfere with my extended user registration form at all. This powerful little plugin opens our marketing possibilities to a whole new dimension. Well done!