I am correct that you can invite anyone to join a ArcGIS Online Group as long as they are a member of and/or have an ArcGIS Organizational Account?
Yes anyone with an arcgis account that is part of an ArcGIS Online organization can join an organization group. There are a few caveats:
The ability to invite users from other organizations to a group in your organization depends on the group settings. For example if you enable the following you will not be able to invite users from outside your organization:
If the user you're trying to invite from another organization doesn't have their profile set to public their username will not appear in the group invitation search results.
Some users may not be able to join external groups based on the privileges set by their administrators.
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