To get a RMCS account, the best thing you can do is write a few comics on the open section, and post the links to the Yahoo group ( http://uk.groups.yahoo.com/group/rmcsforum/ ).
If your comics are good enough, someone will offer you an account. If they are rubbish, they will probably be ignored. Don't let the thought of failure put you off though.
Another way some authors have been testing whether someone is account-worthy is by posting their comic using The Open Marauder. Additionally, they may choose to post your comic under their name to test what others think of your comics.
Alternatively, you could just email your favourite author with lots of sickening flattery. That's how some of us get accounts.