To play Osmo with multiple users, you will first need to register for a myOsmo account at my.playosmo.com. Accounts then allow you to create a profile for each user, linked under that account, that can play within the apps.

Profiles are created from within the apps, but at any point you can edit them (or delete them if you they are no longer required) from your my.playosmo.com home page when you are signed in.