To become a registered user of BGG you need to create your own user name and fill in some details - which can be as few as you want or your entire life history.
To begin registration, click on the word Register just to the right of the BGG Boy, who's on the left near the top of the page.
You need to choose a User Name and a password. The password can be changed at a later date, but the User Name is fixed forever. Make sure you create an appropriate user name.
You may supply false names - if you want to keep your identity hidden, which is OK - but please don't provide false location information, leave it blank instead. You can fill in the rest of the information, including a current email address and then click on "submit" at the bottom of the page.
More info: BoardGameGeek FAQ
tdb: answer question: Can BGG messages be forwarded to regular email?