Writing SQL Insert Function

In short, Functions are not allowed to make any changes to ANY SQL Server object. Stored procedures can. With minor changes, your code is now an SP.

CREATE PROC [dbo].[CreateUser]    (    @Username varchar(20),    @Password varchar(20),    @Email varchar(50),    @PasswordQuestion varchar(30),    @PasswordAnswer varchar(30)    )--RETURNS bit/* datatype */AS    BEGIN        if  (Exists(Select Username from Users where Username=@Username and Password=@Password))            return 1;        else            begin                INSERT INTO dbo.Users (Username, Password,                                        Email, UserId,                                        IsApproved, IsLockedOut,                                        IsOnline, CreationDate,                                        PasswordQuestion, PasswordAnswer) VALUES (@Username, @Password, @Email,                                                                                  1, 0, 0, 0, GetDate(),                                                                                  @PasswordQuestion, @PasswordAnswer);                return 0;            end    ENDGO

And you can call it thus:

exec dbo.CreateUser 'Jim', 'Teddy', 'jim@do.not.email', 'Where', 'Here';