What is the Active Directory structure possibilites with these domain\users?

Currently: What I know - My company primary domain is 'A'.  

So user login looks like this:
1. A\User1
2. A\User2
3. A\User3

However,  when I am in my sql server logins in addition to the 'A\Userx' there is also 'B\Userx' which are external users and not part of the 'A' domain or even part of my company but are part of another organization entirely.
Since I currently am unable to view AD structure on the network please provide how the 'B' domain may be configured to gain access to the Sql Server instance.
rhservanAsked:
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kevinhsiehCommented:
B can be another domain in the same forest, or B can be another domain in a separate forest with a trust relationship.

See http://technet.microsoft.com/en-us/library/cc773178(v=ws.10).aspx
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Steve KnightIT ConsultancyCommented:
Or depending upon how they are showing, is B the SQL servers name, I.e. they are local user accounts on the server itself?
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Vitor MontalvãoMSSQL Senior EngineerCommented:
Domain 'B' is created in your organization?
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