SP Error When Trying To Create User And Assign A Role

I get the following errors associated with trying to create an SP.

Server: Msg 170, Level 15, State 1, Procedure AddFortuneUser, Line 8
Line 8: Incorrect syntax near '@newuser'.
Server: Msg 137, Level 15, State 2, Line 1
Must declare the variable '@newuser'.

Can anyone explain why I have to do a declare.
I suspect I have to issue "declare @newuser sysname" somewhere but I'm not sure why.

The following is the code I'm trying to run.
My intent would be to create a form for the Admin Clerk that would call this SP. That way they can create a generic login. They have an application that allows them to change the password after the fact.
/*
Created for Admin person to allow them to add a basic SQL Login Account
forcing the user to be a member of a specific role 'helmsman'
in a specific database 'Fortune'
*/
CREATE PROCEDURE AddFortuneUser
@newuser char(128)
AS
EXEC master..sp_addlogin @loginame=@newuser, @passwd =substring(@newuser,1,8), @defdb =Fortune
GO
if not exists (select * from dbo.sysusers where name = @newuser and uid < 16382)
EXEC sp_grantdbaccess @loginame=@newuser, @name_in_db=@newuser
GO
exec master..sp_addrolemember @rolename ='helmsman', @membername =@newuser
GO