Hello.
Thank you for your clarification! Do you know when the group was created, was "Shared update group" enabled? Here is a shared update article. Depending on that, we might have 2 different solutions:
I found that "in some instances when attempting to invite members outside the organization to a group without the shared update capability, members with Viewer roles are not returned in the search results."
What we can do to resolve this is:
For groups with the shared update capability, change the member role, or create a new group without the shared update capability to enable adding members with Viewer roles to the group
To add members with Viewer roles to a shared update group, change the member's role to User, Publisher, or Administrator. Alternatively, to add members with Viewer roles from within the organization to a group, create a new group, and select the Only their own items option under the What items in the group can its members update? setting.
For groups without the shared update capability, toggle the 'Only search in <organization name>' option on, or create and assign a custom role to the members to enable adding members with Viewer roles to the group
- When searching for members with Viewer roles within the organization to add to an existing group without the shared update capability, toggle the Only search in <organization name> option on.
- To add members with Viewer roles outside the organization to an existing group without the shared update capability, create and assign a custom role, or modify the custom roles for the members based on the default Viewer role.
Additionally, for the error regarding, "The invited user has insufficient privileges to join external groups":
- Log in to ArcGIS Online, and click Organization > Settings > Member Roles.
- Click the Create Role option, and create a new role for the user. Refer to ArcGIS Online: Configure member roles for more information.
- Click Import settings from an existing role or template, and select a template role (for example, Data Editor). Click Import settings.
- Under Role privileges, check the Join external groups check box to enable the user to join a different ArcGIS Online organization group. Click Save role > Save.
- Click Assign Roles > Assign to assign the new role to the user. Refer to ArcGIS Online: Roles for more information.
Resources:
Please try the above, and I hope that can help resolve your issue!
Saka D
ArcGIS Analyst | Esri