Users are stored within the Saiku H2 database, more configuration options will be coming in Saiku 3.x, although you can change the Spring configuration files now to point to a different database an administer the users outside of the Saiku Administration console.
To add a user to Saiku login as an Administrator and click on the "A" logo on the toolbar. Under user management select Add User.
Now enter a username (mandatory), and optionally an email address. You can then enter a user password, and add some roles to the new user to allow them to see certain files and folders. Finally click Save User and the user will be added to the database.
To remove a user, select the existing user from the list on the left hand panel of the Administration Console. You can then click the Remove User button to delete the user.
To edit a user, change roles, password etc, you can select the user from the list on the left and then make adjustments to the users details in the main edit panel. Once you have made your desired changes you can then click the Save button to save the users details.