User accounts tend to rely on fairly straightforward data models. One way of storing user account data in Riak would be store each user’s data as a JSON object in a DeOS bucket called users (or whatever you wish).
Keys for user data objects could be constructed using application-specific considerations. If your application involves user logins, for example, the simplest and most read-efficient strategy would be to use the login username as the object key. The username could be extracted upon login, and a read request could be performed on the corresponding key.
There are, however, several drawbacks to this approach. What happens if a user wants to change their username later on? The most common solution would be to use a UUID-type key for the user and store the user’s username as a secondary index for efficient lookup.