Hello Fridah,
Well, the quick answer is that if you have users within your Forest, but from another domain, that means that their sign-in name probably has a different domain than the users in your Lync 2013 domain. For example, if the domain that your Lync 2013 deployment
is in is called domainA.com, then likely the users in this domain sign in to Lync with an address like user1@domainA.com. Further, the Lync 2013 deployment was probably only setup with 1 SIP domain: domainA.com. Now, if a user from the other domain, lets call
it domainB.com, has an address of user2@domainB.com, and they try to sign into Lync (assuming their client is even finding the Lync server to begin with), they will not likely be able to log in because domainB.com is not setup as a SIP domain within Lync 2013.
Obviously I am making a lot of assumptions above about what your setup might look like, but other things come into play like if you have all the proper DNS records setup for an assumed second domain address to login. This could actually be quite a number
of things, and is very difficult to narrow down without many more details about your deployment, and if you deployed the Lync 2013 installation with multiple SIP domains in mind or not. However, if this brief bit of information does not really point you in
a direction that gets you working, I would suggest opening a new thread with full details about how you designed the installation with this second SIP domain in mind, and what the environment looks like (as it is not really related to the question that this
thread was opened for).