We have TFS 2015 installed on a server and are looking at having a 3rd party connect and help with the workload. With this, we do not want to create an Active Directory user for them unless we absolutely have to. My question is, is there a way that TFS 2015 supports users outside of an active directory environment and is there a way to do dual authentication for those users?
I did find this article and was not sure if it applies to TFS 2015 as well. Team Foundation Service is not an option for us as an option either.
No, you will have to create an AD account for every external user.
If you move to VSTS (there is a solid migration route), you can use AAD and add the venders AAD or MSA accounts to your AAD and give them access. The feature is called Guest accounts. There are no good reasons not to move to VSTS. It's more secure and has more features... And you dont have to upgrade or add disk space ever again.