List of all SQL Server users

select distinct LOGINNAME 
  from dbo.SYSUSERS u, master.dbo.SYSLOGINS 
  where U.NAME = USER

List of Users with Database Access

select distinct   
  LOGINNAME = (case when (O.SID = 0x00) then NULL else L.LOGINNAME end),
  user_name(O.GID),
  O.UID,
  O.HASDBACCESS 
from dbo.SYSUSERS O
  left join (select SID, LOGINNAME, 1 as MATCHED from MASTER.dbo.SYSLOGINS) L on L.SID = O.SID 
where ((O.ISSQLROLE != 1 
        and O.ISAPPROLE != 1 
        and O.STATUS != 0 
        and MATCHED is not null) 
       or (O.SID = 0x00) 
       and O.HASDBACCESS = 1) 
       and O.ISALIASED != 1 
order by O.LOGINNAME