SQL SERVER equivalent for Oracle DBA_USERS

The sys schema is your friend for these kind of queries. Specifically sys.sysusers will give you the list of all Users of the current database.

You can also get a list of all server Logins using sys.syslogins.

In SQL Server, a server Login is first created, and then mapped to the databases that Login has access to. Each database a Login is mapped to has a User created within that database for that Login.