The Tufts enterprise license for Github does accommodate the ability to create additional Github organizations, but there are many caveats associated with this and it is usually not recommended to do so. To quote the Github best practices:
In general, GitHub recommends minimizing the number of organizations you create. Having fewer organizations encourages greater collaboration and innersourcing, which increases efficiency. In fact, many businesses are best served by a single organization, for the following reasons.
The reasons they cite are (also quoted from Github):
It's easier to find resources within a single organization, as there's only one place to search.
It's easier to communicate within a single organization, as @-mentions only work between members of the same organization.
Being part of a single, large organization where anyone and anything is accessible fosters collaboration and loyalty, whereas being separated into smaller organizations can make teams more isolated.
The creation of a new Github organization underneath the Tufts enterprise also carries an additional management burden regarding the care and maintenance of that org, which will be the responsibility of the requesting party, and includes things such as:
setup of authentication, such as Shibboleth or Active Directory
maintenance of related AD groups
specifically managing access for any cross-group collaborators
contacting Github support in the event of issues