Hiya, so I'd like to incorporate Codoforum's login system with my main website's login database. How can I do this?

Thanks!

refer this tutorial http://codoforum.com/documentation/implementing-codoforum-sso

refer this tutorial http://codoforum.com/documentation/implementing-codoforum-sso

If I have users already signed up, will they be able to login with the same accounts on both sites - or will they be separate logins?

They will be able to login with the same accounts because they will be logging in with your website login form and the codoforum login form will not be available.
As soon as they login on your website, they will be automatically logged in on codoforum.