Same concept as just logging in. Just use a set of tables, store the info you want to be specific to each client in rows as variables (set up relationships between login/password and content). When a client login is checked, get the variables from the the table where (client).
Watch out, though. If you do a select/where from the login form, check the address bar of the browser- it may display the variable used to select the record. Use an include to get around this security problem.
Best of luck