From Textpattern CMS User Documentation
Before getting a wiki account, you must already have an account in the Textpattern Support Forum. Your forum username will be used for your wiki username too.
When you have your forum account, contact one of the following people and provide them with your forum username and an email address:
They will create a new account for you following the help:Creating New User Accounts process, and your account details will be emailed to you automatically from the wiki. Change your password upon first login!
List of Wiki Users
Before requesting an account, make sure you don't already have one you forgot about by checking the Wiki Users List