To mess with the default registration system may not be a good idea as it may cause unwanted problems during the upgrades. The best option in my opinion will be to consider using a plugin for this purpose whether premium or one of the free membership plugins from the wordpress.org repository. The only difference will be that the paid ones will pay personal attention but at a price.
Your site has a server stability issues as it hanged while i was browsing. You may wanna check that with your host. It seems like a business site so, you may also consider to put your call to action "SIGN UP FOR A PLAN" button just right below the slider, which will give it more visibility. Right now one has to crawl down a bit to see the button. Lots of links are not working, but I guess it is a part of the development.