Oracle Tables Visible to Other Users

List all users who have been assigned a particular role

-- Change 'DBA' to the required role
select * from dba_role_privs where granted_role = 'DBA'

List all roles given to a user

-- Change 'PHIL@ to the required user
select * from dba_role_privs where grantee = 'PHIL';

List all privileges given to a user

select
  lpad(' ', 2*level) || granted_role "User, his roles and privileges"
from
  (
  /* THE USERS */
    select 
      null     grantee, 
      username granted_role
    from 
      dba_users
    where
      username like upper('%&enter_username%')
  /* THE ROLES TO ROLES RELATIONS */ 
  union
    select 
      grantee,
      granted_role
    from
      dba_role_privs
  /* THE ROLES TO PRIVILEGE RELATIONS */ 
  union
    select
      grantee,
      privilege
    from
      dba_sys_privs
  )
start with grantee is null
connect by grantee = prior granted_role;

Note: Taken from http://www.adp-gmbh.ch/ora/misc/recursively_list_privilege.html

List which tables a certain role gives SELECT access to?

-- Change 'DBA' to the required role.
select * from role_tab_privs where role='DBA' and privilege = 'SELECT';

List all tables a user can SELECT from?

--Change 'PHIL' to the required user
select * from dba_tab_privs where GRANTEE ='PHIL' and privilege = 'SELECT';

List all users who can SELECT on a particular table (either through being given a relevant role or through a direct grant (ie grant select on atable to joe))? The result of this query should also show through which role the user has this access or whether it was a direct grant.

-- Change 'TABLENAME' below
select Grantee,'Granted Through Role' as Grant_Type, role, table_name
from role_tab_privs rtp, dba_role_privs drp
where rtp.role = drp.granted_role
and table_name = 'TABLENAME' 
union
select Grantee,'Direct Grant' as Grant_type, null as role, table_name
from dba_tab_privs
where table_name = 'TABLENAME' ;

If you actually have different databases (and it sounds like you do), you can't grant permissions for users in one database to access data in another database. You'd need to create a database link in the EmployeeDB database that connects to the ContractorDB database. That database link can either use a fixed username and password in the ContractorDB database (i.e. create an EmployeeUserRemote user in ContractorDB with a password that doesn't change that can be hard-coded into the database link definition) or that database link can be a CURRENT_USER database link in which case the username and password would need to match in the two databases for every user that wants to use the database link. Queries that use the database link log in to the remote database (ContractorDB) as the specified user and have the privileges of that user in the remote database.