Can you search for/invite members of another organization to a group in your organization if they aren’t an organization on ArcGIS Online?

[DarkoRadiceski]

ATM, it seems as if the only way to search for/invite people to a group in your organization is if both organisations are:
o Organisations in ArcGIS Online
o On version 10.6 of portal/server or greater?

[KellyGerrow]

Hi Darko,

In order to be invited to an ArcGIS Online Group, the user must have a username and password in an ArcGIS Online organization and have their profile visible to everyone. It isn't possible to add an ArcGIS Enterprise (Portal) member to an ArcGIS Online group.

In ArcGIS Enterprise 10.5.1 and 10.6, there is the capability to set up ArcGIS Enterprise to ArcGIS Online Collaboration. This will allow members in an ArcGIS Online Organization and ArcGIS Enterprise deployment to access copies of the same data and web maps through synchronization. It will not give the Enterprise user access to the Online group.

Distributed collaboration—ArcGIS Online Help | ArcGIS 

Let me know if you have any questions,

-Kelly