As far as 'einrollen' is concerned: delete it from your mental hard drive ;-) It literally means to roll up something (or 'to approach' in the case of a train entering the station). I guess you confused it either with the English 'to enrol' or the German 'einschreiben'. Generally speaking, 'anmelden' and 'registrieren' are pretty much equivalent. On the internet, however, 'registrieren' is normally used to create a new account and 'anmelden' to log into an existing account. As far as the English words are concerned: 'log in' or 'sign in' are used for an existing account on the internet. 'Sign up' is to create a new account.