We are using Microsoft Team Foundation Server 2018 for the development of internal tools. May external users support our development processes?

External users may also access the Team Foundation Server 2018. To do so, the external organization must supply them with one of the following Visual Studio licenses:

– Visual Studio Enterprise Subscription
– Visual Studio Enterprise – annual
– Visual Studio Enterprise – monthly
– MSDN Platforms
– Visual Studio Test Professional Subscription

Moreover, the organization which licensed the Team Foundation Server must assign a Team Foundation Server CAL to each external user. The accesses on the Windows Server – if used for running the Team Foundation Server – must also be covered.

Source: Whitepaper_Microsoft_Visual Studio 2017_Team Foundation Server 2018_as is 2018-03-02