Single user registration for multiple sites – How to?

(thanks wordpress for great service and members btw)

I have a website called gibbly.net
(runescape tips and guides, etc.)

Then I have sub domains such as runescape.gibbly.net

Is there a way to connect the registration so that you only have to make one account and it’s active in every sub domain?

I have little to no experience in coding.

Thanks in advance for the help.