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This allows you to assign group permissions to global roles without those people having to be a member of a group. Really useful for administrator type roles that need to be able to perform specific operations on all groups without them having the extremely powerful "bypass group access" permission.
API breaks
The main one to note is the removal of GroupContentEnablerInterface::getEntityForms(). Other than that, some classes disappeared because they are now in core 8.1.x.