Detailed Lync Settings
Lync is a tool for sharing and collaboration. Some projects need people who are inside and outside of the organization. What are the permissions?

3. Try It: Edit the Lync Settings
The Office 365 Admin Center is still open.
Go to External Sharing->Lync.

What Do You See? By default, Lync users can invite peopel outside of their organization.

The permissions can be defined as:
White Hat (good guys in domains that are allowed) or

Black Hat (bad gus who are in blocked domains.)

What Else Do You See? Lync can also communicate with Skype users. By default this is already turned on.

Click Save. That will do it.

Home->Admin->Office 365->External Sharing->Lync