While anyone may contribute to Sensei's Library anonymously, having a user name lets you get credit for your work and makes it easier to others to identify you.
A user name may consist only of letters and numbers, and must contain at least one letter. Spaces are not allowed. For example, "ArnoHollosi" is a valid user name, but "Arno Hollosi" and "123" are invalid.
You can set your user name for edits at User Preferences.