When you have been invited to access an AdWords account, you will be sent an invitation from Google to your email address. There will be a link in the email which will take you to the standard page to accept your Google invitation.
If you haven’t previously signed up for a Google product before, you will be asked to create a new Google account in the backend with your email address. A Google account will then hold all the Google products you have signed up for with your email address in a single location.
If your email address is already associated with a Google Account (i.e. if you sign into ANY Google products using your email address), simply select “I have an email address and password I already use with Google services like AdSense, Gmail, Orkut, or iGoogle” followed by “I’d like to use my existing Google account to log into AdWords”, and you should be able to access the account.