2 Domain Users to access sane AOS

I have a quick question.

I want to allow the users of 2 different domains to access the AOS server.

i.e. I installed my AOS & Database server on DOMAIN1 the users from DOMAIN1 can access it no problem but i have another domain DOMAIN2 i want to allow the users of this DOMAIN2 to access the same AOS server.

How can i allow the users of DOMAIN2 to access the same AOS server.

Any help of material that can help me in achieving this is highly appreciated.

Thanks in advance.