well, i'd start with implicit users that come from windows groups:

select *

from master.sys.server_principals

where type_desc = 'WINDOWS_GROUP'

from there, for each name in the list, you can use a built in extended stored procedure which queries Active directory to enumerate all the users in the group

EXEC master..xp_logininfo

@acctname = 'disney\authenticatedusers', --an example windows group, where my domain is "disney"

@option = 'members' -- show group members

EXEC master..xp_logininfo

@acctname = 'Builtin\Administrators',

@option = 'members' -- show group members