Request account
From btrfs Wiki
Complete and submit the following form to request a user account.
Once the account is approved, you will be e-mailed a notification message and the account will be usable at login.
If you do not get a reply within a few days, your request has probably been rejected as suspicious (i.e. spam) or no wiki admin had the time to review your request yet (happens occasionaly). Just drop a mail on linux kernel btrfs mailing list with an appropriate subject to get attention.