My understanding is that the special relationship between an AGOL organization and it’s related Hub Community organization is supposed to allow Hub Community accounts to be a member of AGOL “update” groups.
Currently however, Hub Community accounts can only be invited to Hub Core Team “update” groups, and can be invited only by using a Hub initiative’s Team page. That’s a problem because we have “update” groups used for multiple initiatives, and collaboration groups for which an “initiative” app has not been created. One such use case is having Esri solution engineers assist with projects.
When you try to invite an account to an Update group, the option to invite people outside the organization is grayed out, with a message saying:
“For groups that allow other group members to update all items in the group, you can only invite members of your organization”.