How do I license the access of another company to my company-owned Windows Server 2012 R2?
There are two ways to license the access of external (not your employees or contractors) users to Windows Server. Either the external users/devices are known and they get assigned the corresponding Windows Server CALs, or an External Connector license is necessary. The External Connector license is assigned to the server, which is being accessed directly or indirectly, and allows any number of external users/devices to access the Windows server.
It should be noted that a CAL may also be necessary for some applications installed on the server.
Source: Microsoft Volume Licensing Blog as is 2017-03-08 (https://blogs.technet.microsoft.com/volume-licensing/2014/03/10/licensing-how-to-when-do-i-need-a-client-access-license-cal/)