The private or organizationally shared ArcGIS Online username is not searchable if it contains keywords that are part of a longer, publicly shared ArcGIS Online profile username.

Solución alternativa

Workaround 1:

1. Replace USERNAME in the following URL with the username to invite to your group and paste it in a browser: https://arcgis.com/sharing/rest/community/users/USERNAME?f=json
2. Copy the user id from the results page, and search for it in ArcGIS Online (Group > Invite Members). For example, '"id":"USERID"'.This should return the searched user to add to the group.

OR

Workaround 2:

1. Add an additional character to 'online'. For example: onlinea/aonline/onlinea_001.

Pasos para reproducir