You'll probably just get a lot of dumb questions - like this one: If I am signed in to the message board, am I signed into the site. Maybe a FAQ page about signing in??
No they are two separate things (but you can use the same info)
Hmmmm.... I went out to register and it said ERROR: This username is already registered. Please choose another one. So I went out to the sign in page and it says the password for that username is invalid. Tried it four or five times.
Did you try resetting the password?
If not that, are you sure no one else has the name you're trying to get?